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Atomic Weight of Hydrogen, history






In the history of chemistry there is no more interesting chapter than that recounting the controversy waged regarding the choice of the best standard of atomic weight. The various determinations of the atomic weight of hydrogen are of special interest and importance, because for many years this element was the arbitrary standard. In the early years of the nineteenth century, it was first selected for this purpose by John Dalton, its atomic weight being taken as unity and the atomic weights of the other elements being expressed relatively to this value. Dalton's choice was determined by the fact that hydrogen is the lightest of the elements, but the weak point in selecting it as standard lies in the fact that the atomic weights of most of the other elements cannot be determined directly with reference to hydrogen. This defect was noted by Berzelius and by Wollaston, who adopted oxygen as the basis of their atomic weight systems. Since most of the elements combine with oxygen, it furnishes a useful standard of atomic weight, and any revision of the ratio hydrogen: oxygen does not necessitate recalculation of the whole of the atomic weight table. Wollaston chose for his fundamental atomic weight O=10, and was followed by later chemists, such as Despretz in 1826, Kiihn in 1837, and Cauchy in 1838. Berzelius objected to Wollaston's value for oxygen on the ground that it necessitated expressing the atomic weight of hydrogen by a fraction, and founded his table on the basis O=100. The standard O=1 was adopted by Meinecke in 1817, by Bischof in 1819, and also by Thomson.

Dalton formulated water as HO, but Berzelius considered that its formula should be H2O, since it can be produced from two volumes of gaseous hydrogen and one volume of gaseous oxygen. Convinced of the impossibility of determining atomic weights accurately, Gmelin and his adherents in 1826 and later years advocated the adoption of a set of equivalents, that of hydrogen being taken as unity, and that of oxygen as 8 on the basis of Dalton's formula for water. According to Berzelius's view, the atomic weight of hydrogen compared with oxygen as 100 was 6.24, but in deference to the views of Gmelin he introduced the conception of double atoms, denoted by a horizontal line through the symbol of the element, the corresponding numbers being known as " Berzelius's atomic weights." In the table based on this compromise the ratio H:O was 12.48:100 in Berzelius's notation, or in round numbers 1:8 in that of Gmelin.

Experiments by Dumas and by Erdmann and Marchand in 1842 fixed the ratio H:O as 12.5:100=1:8.00, and it remained unaltered until 1860. In 1858-1860 Cannizzaro proposed adopting for oxygen the atomic weight O=16, and, in accordance with his suggestion, the atomic weights of a number of the elements were taken as double their equivalents. In 1860 Stas determined the atomic weight of hydrogen to be 1.005 (O=8), in terms of Berzelius's system; and in 1865 he gave the value as 1.0025 compared with O=16, in accordance with modern conceptions.

Although neither of these numbers agrees with the present accepted value, it is noteworthy that Stas was among the first to suggest employing O=16 as the standard, a system adopted by him almost exclusively in his published work. He did not utilize the standard H=1, beyond stating that in comparison with it the value for oxygen could not exceed 15.96, and giving a number of atomic weights calculated on this basis; but the atomic weight of hydrogen as unity proved very attractive, and was always employed by many chemists, although some adhered to Stas's table, calculated from O=16. In his book on atomic weights, published in 1882, Clarke employed chiefly the values H=1 and O=15.96 as the basis of calculation, although he also used O=16 as standard. In a treatise on atomic weights, published in 1883, Lothar Meyer and Seubert adopted the ratio H:O=1:15.96: but they also tried to introduce a system based on the value O = 1.

In 1882 Stas announced a new value for the atomic weight of hydrogen, 1.01 (O=16), and in the following year Marignac urged without success the adoption of Stas's original value O=16 as the standard. In 1885 Ostwald criticized Stas's conclusion that the value O=16 necessitates for hydrogen a value higher than unity, and employed the ratio H:O=1:16 as the basis of his atomic weights.

Such was the condition of the subject in 1887, when the publication of results obtained by Cooke and Richards, and by Keiser, shed a new light on the matter. Their work, confirmed by that of Rayleigh and of Crafts, proved the ratio H:O=1:15.96 to be inaccurate, and indicated 15.87 as the atomic weight of oxygen. Lothar Meyer and Seubert still held out for the old ratio, and advocated the employment of H=1 as standard.

In 1888 papers were published almost simultaneously by Brauner, Ostwald, and Venable advocating the adoption of the standard 0=16, in accordance with Marignac's suggestion made in 1883. In 1889 the German Atomic Weight Committee was appointed at the instance of Brauner, and in 1906 the International Committee on Atomic Weights, constituted in 1900, took the decisive step of adopting O = 16 as the sole standard. The atomic weight of hydrogen given in the current table is 1.008.

A summary of the literature of the dispute has been given in a paper by Kiister, and in another by Brauner.


Methods of determining the Atomic Weight

From the historical standpoint it will be most interesting to consider the various determinations of atomic weight in chronological order, giving the values according to both the oxygen standard and the hydrogen standard, the values for the older determinations being expressed in the notation of the period. For convenience in treatment, the methods described will be classified in two divisions -
  1. Gravimetric methods involving the synthesis of water by various processes.
  2. Physical, physico-chemical, and volumetric methods employing gaseous hydrogen and oxygen.

Gravimetric Methods

In 1819 Berzelius and Dulong reduced a weighed quantity of cupric oxide at red heat in a current of hydrogen, and weighed the water produced. The loss in weight of the cupric oxide gave the weight of oxygen in the water formed, whilst the difference between the weight of the water and that of the oxygen gave the weight of the hydrogen. In three experiments the ratio H:O was found to be 1:16.124, 15.863, and 15.106 respectively, the mean being 1:16.031. In Berzelius's system the corresponding value for the ratio H:O is 6.2379:100, the atomic weight for hydrogen employed by Berzelius being 6.24. Dulong and Berzelius determined the densities of the two gases, the ratio being DH:DO = 0.0688:1.1026, and the corresponding atomic weight of oxygen 6.2398, which agrees well with that calculated by the gravimetric method. The ratio is

H:O=1:16.031 = 0.99804:16.

To correct the error caused by weighing in air, this value has been recalculated, introducing a correction for vacuum, and the value H=6.2915 obtained. The corresponding ratio is

H:O=1:15.894=1.0067:16,

a close approximation to the value accepted at the present day.

In 1842, in co-operation with Stas, Dumas repeated the work of Berzelius and Dulong, employing many precautions to ensure the purity of the materials. Without applying any corrections, the value of the ratio found was

H:O=1:15.958=1.0026:16.

Subtracting the weight of the water formed from the air dissolved in the dilute sulphuric acid employed to generate the hydrogen, Dumas obtained the ratio

H:O=1:15.988=1.00012:16.

The ratio of the two equivalents is 1:8, from which the ratio of the atomic weights in accordance with modern views is

H:O=1:16.

This value was for many years adopted as the atomic weight. Dumas's method has been subjected to searching criticism at various times. Berzelius objected to it on the ground that the air employed to displace the hydrogen at the end of the experiment dissolved in the water formed, thus augmenting the value for the atomic weight of hydrogen, and diminishing that for oxygen. Melsens pointed out that a similar error resulted from occlusion of hydrogen by the reduced copper, the weight of the oxygen being consequently too low. The chief source of error in Dumas's method was the presence of occluded gases in the cupric oxide employed, a point noted by Richards and Rogers. During reduction these gases were given up by the oxide, the consequent loss of weight being reckoned as oxygen, whereas part of it was due to other occluded gas. The result, 15.96 to 15.99 (H=1), indicates too low a value for the atomic weight of hydrogen and too high for oxygen.

In 1842 Erdmann and Marchand followed closely the lines marked out by the earlier work of Berzelius and Dulong, and especially that of Dumas. The precautions taken and the sources of error overlooked were similar to those characteristic of Dumas's work. In some of their experiments the copper oxide was prepared from metallic copper, and in others by heating copper nitrate. From one set of four experiments the mean value obtained was 6.2742 (O=100), from which follows the ratio

H:O=1:15.938 =1.0039:16.

In another set of experiments efforts were made to determine the amount of air occluded on the surface of the copper oxide and the metallic copper, but the results obtained varied between wide limits. Four experiments were then made, each tube and its contents being weighed in vacuum before and after the experiment. The mean of the four results obtained was 6.2459 (O=100), corresponding with the ratio

H:O=1:16.015 =0.9993:16,

or, in round numbers, 1:16, a value regarded by Erdmann and Marchand as fully supporting the work of Dumas.

From his examination of the relation of silver to silver nitrate and ammonium chloride, Stas in 1860 found the value 1.005, the " atomic weight " (according to the nomenclature of the period) of oxygen being taken as 8. The corresponding ratio is

H:O=1: 15.924=1.005: 16.

Five years later, Stas stated that, although there was no certainty as to the exact value of the ratio of the atomic weights of hydrogen and oxygen, he was convinced by the results of the various researches on the composition of water, on the density of hydrogen and of oxygen, and on the ratio of ammonium chloride and silver, that if the atomic weight of hydrogen were taken as unity, that of oxygen could not exceed 15.96, so that

H:O=1: 15.96=1 0025: 16.

This number differs from the accepted atomic weight more than the first result of Stas.

In the experiments of Thomsen, published in 1870, the weight of water formed by the interaction of a known volume of hydrogen with cupric oxide and with oxygen was determined, the ratio calculated being

H:O=1: 15.9605=1.00247:16.

Thomsen's result was vitiated by his employment of an inaccurate value given by Regnault for the weight of a litre of hydrogen, 0.08954 gram. Clarke recalculated the ratio, employing the modern value for the weight of a litre of hydrogen, his result being

H:O=1:15.91=1.0057:16.

The application of a new combined gravimetric and volumetric method to the determination of the ratio of silver to ammonium chloride and also to ammonium bromide furnished Stas in 1882 with another basis for ascertaining the atomic weight of hydrogen. His mean result, recalculated with the atomic weights Ag=107.880, Cl=35.457, Br=79.916, and N=14.010, gives the ratio

H:O=1:15.793=1.0131:16.

The value for hydrogen thus obtained was too high, in contrast with the low ratios previously published by Stas.

By oxidation of a known volume of hydrogen, van der Plaats in 1886 obtained a mean value corresponding with the ratio

H:O=1:15.95=1.003:16.

The investigation of Cooke and Richards, begun in 1882, marks the opening of a new era in the history of the determination of the atomic weight of hydrogen, and after the application of necessary corrections the results yield a value identical with that obtained by Morley in 1895. The hydrogen employed was prepared from zinc and hydrochloric acid, by electrolysis of dilute hydrochloric acid with a zinc-amalgam anode, or by the action of aluminium on potassium hydroxide; and so purified by contact with potassium hydroxide, calcium chloride, sulphuric acid, and phosphorus pentoxide that spectroscopic tests failed to reveal the presence of any extraneous substance. The hydrogen was weighed directly, and oxidized by copper oxide prepared from pure electrolytic copper, the water formed being absorbed by phosphorus pentoxide and weighed, all weighings being reduced to vacuum. It was noted by Mendeleeff, and later by Agamennone, that the volume of an evacuated glass globe is diminished by atmospheric pressure, and at the suggestion of Lord Rayleigh a correction for this diminution was introduced by Cooke and Richards into their calculation, the hydrogen having been weighed in an elongated, cylindrical glass flask. The ratio obtained was

H:O=1:15.869=1.00826:16.

An observation of Mendeleeff that a change of pressure of one atmosphere produces a corresponding change in the volume of water, suggested to Brauner the necessity for applying a further correction to the calculation of Cooke and Richards, to eliminate an error introduced by their method of determining the capacity of their glass vessel. The ratio as recalculated by Brauner is

H:O=1:15.879=1.00762:16.

The method employed by Keiser in 1887 involved weighing hydrogen after occlusion by palladium, and weighing the water produced by its combustion. The mean of three experiments gave the ratio

H:O=1:15.864=1.0086:16.

In a later paper Keiser published a new value for the ratio, based on the mean of ten experiments:

H:O=1:15.9514=1.00307: 16.

The process employed by Rayleigh was an improved modification of that of Fourcroy, Vauquelin, and Seguin, weighed quantities of pure hydrogen and oxygen being mixed together, the mixture introduced into a eudiometer in successive steps, exploded electrically, and the residual gas analysed. By applying Rayleigh's correction for the diminution in volume of the evacuated glass flask, and consequent change in weight, the ratio obtained was

H:O=1:15.89=1.0069: 16.

Almost simultaneously with Rayleigh's paper appeared one by Noyes. The noteworthy feature of Noyes's investigation was the simple form of apparatus employed, the interaction of the hydrogen and copper oxide and the weighing of the resulting water being carried out in the same vessel. In the experimental work minute care was taken to eliminate sources of error, but the results were adversely criticized by Johnson as being too low. His objection was based on observations made during his own earlier work, in which he noted the occlusion of hydrogen by reduced copper, a fact previously mentioned by Melsens. Noyes considered that the precautions taken eliminated this source of error, and calculated from the mean of twenty-four experiments the ratio

H:O=1: 15.897=1.0065: 16.

The principle of the method employed by Dittmar and Henderson is similar to that of Berzelius and Dulong's method, the weight of the hydrogen oxidized by the cupric oxide not being determined directly, but calculated indirectly from the quantity of water produced. Two series of experiments were made, and after completion of the first series the authors observed a source of error in their method, due to the contamination of the hydrogen by sulphur dioxide in consequence of reduction of the sulphuric acid used for drying. Dumas also noticed the same difficulty, and tried to prevent the reduction to sulphur dioxide by cooling the sulphuric-acid drier with ice. In their second series of experiments Dittmar and Henderson removed the sulphur dioxide by passing the dried hydrogen over fused potassium hydroxide. The ratio calculated from the mean result of both series of experiments was

H:O=1:15.865=1.0085:16,

the mean value for the atomic weight in the first series being 1.00850, and in the second 1.00848.

Leduc, in 1892, also synthesized water by oxidizing hydrogen with cupric oxide, the mean of two experiments giving the ratio

H:O=1:15.88=1.0075: 16.

The classical research of Morley, a masterpiece of skill, ingenuity, and patience, carried out by a master of experimental method, consisted of four parts. In the first he determined the weight of a normal litre of oxygen; in the second the value of the same constant for hydrogen; in the third the relative proportions by volume in which, under normal conditions, hydrogen and oxygen unite to form water; and in the fourth the amount of water formed by the union of weighed quantities of oxygen and hydrogen. Since they are of a physico-chemical nature; the fourth part involves a gravimetric process, and is accordingly described here.

Morleys apparatus
The atomic weight of hydrogen. Morleys apparatus for producing and weighing the water.
In Morley's experiments the weight of oxygen contained in two globes was determined, and a quantity of hydrogen was weighed while absorbed in palladium. The two gases were combined, and the weight of water produced was ascertained. The gases were brought into contact at two platinum jets enclosed in a small glass apparatus (See Fig) previously exhausted and weighed. After the combination, the residual gas in the combustion chamber and the connecting tubes was extracted by means of a Toepler pump, measured, and analysed. The combustion chamber, the oxygen globes, and the palladium-hydrogen tube were again weighed. The difference between the original weights of oxygen and hydrogen and those of the gases analysed gave the quantities combined in the combustion chamber. The gain in weight of the combustion chamber corresponded with the amount of water produced, and should have been equal to the sum of the weights of the gases consumed. The observed difference was due to experimental errors, and indicated the degree of accuracy of the operation.

In most of the experiments the volume of hydrogen employed was between 42 and 43 litres, and the weight of water produced was about 34 grams. The proportion of uncombined gas varied between one six- hundredth and one ten-thousandth of the total amount. Each synthesis was complete in about one hour and a half.

The gases entered the combustion chamber at the jets a, and combination was initiated by sparking across the gap between the wires ff. The two tubes bb were filled with phosphoric anhydride kept in place by asbestos, the oxide serving to prevent the escape of any traces of water formed. The joints cc were ground to fit corresponding joints connecting the apparatus through other phosphoric-anhydride tubes to the sources of oxygen and hydrogen. The tubes bb were sealed at d and e, notches indicating the points of subsequent fracture. The hooks at the ends of the apparatus facilitated the hydrostatic weighing for determining its volume.

The whole vessel was exhausted to one ten-thousandth of an atmosphere, sealed off at g, and weighed against a similar counterpoise. The points d and e were then broken off, and the tubes bb connected to the oxygen and hydrogen supplies through a " manipulator " for the admission of the gases as required, and for continuous observation of their pressures and of the pressure in the combustion chamber. This device also precluded the possibility of communication between the sources of oxygen and hydrogen.

During the combustion, the apparatus was immersed in a bath of cold water. The lower part of the combustion chamber was ultimately cooled to - 18° C., the uncombined gases extracted by the pump, and the apparatus closed by fusion at hh. It was then dried and re-weighed with the pieces detached by fusion or fracture.

The hydrogen was prepared by the electrolysis of dilute sulphuric acid, and the oxygen by heating potassium chlorate. Each gas was purified and dried carefully. The phosphoric anhydride was proved to have been without action on the oxygen, and the freedom from moisture of the hydrogen evolved from the palladium was also demonstrated. The residual gas sometimes contained traces of nitrogen, derived from the oxygen, and due allowance was made for this impurity. A similar correction was applied for traces of carbon dioxide formed by oxidation of a minute proportion of organic matter in the asbestos packing of the tubes bb.

For twelve experiments the ratio of the hydrogen to the oxygen employed gave the mean value

H:O=1:15.8792±0.00032,

and for eleven experiments that of the weight of hydrogen used to the weight of water produced gave the mean value

H:O=1:15.8785±0.00066.

Morley chose 15.8790 as the mean of these two ratios, which is also their "weighted mean." He concluded that the atomic weight of oxygen, compareda with hydrogen as unity approximates very closely to O=15.8790. The value of the ratio calculated from Morley's results is therefore

H:O=1:15.8790=1.00762:16.

In 1894 Thomsen employed an indirect method for determining the atomic weight of hydrogen, involving a knowledge of the atomic weights of chlorine and nitrogen. The method was based on the direct combination of weighed quantities of ammonia and hydrogen chloride when passed together into water until the solution was almost neutral, the slight excess of ammonia being determined by titration. The result obtained does not accord with that of Morley.

Thomsen's second method of investigation embodied a novel principle. Weighed quantities of aluminium were brought into contact with a solution of potassium hydroxide, and the evolved hydrogen dried and weighed, the results giving the value of the ratio H:Al. Another series of experiments was made, the evolved hydrogen being combined with oxygen, and the water formed weighed, the data obtained being employed to calculate the ratio O:Al. From the results of the two sets of experiments, the ratio of the atomic weights was calculated as

H:O=1:15.869=1.00826:16.

In 1897 Thomsen revised the results obtained in this research, introducing a correction for the reduction of volume accompanying the solution of the aluminium in the potassium-hydroxide solution. The amended ratio is

H:O=1:15.8685 =1.00829:16.

In 1898 Keiser caused a known weight of hydrogen occluded in palladium to combine with oxygen, and determined the weight of the resulting water, the whole process being carried out in one apparatus. The mean of four experiments gave the ratio

H:O=1: 15.880=1.00756: 16,

a close approximation to Morley's value.

The principle of the investigation carried out by Noyes in 1907 was similar to that adopted by him in the research of 1889-1890, hydrogen being oxidized by copper oxide, and the water formed being weighed in the oxidation apparatus. Five series of experiments were made, but the results of the first were not employed in the final calculation, being vitiated by retention of water in the copper oxide. The mode of procedure was modified for each series of experiments, the mean of twenty-five observations giving the ratio

H:O=1:15.8751=1.00787: 16.

II. Physico-chemical Methods

The physico-chemical methods consist essentially in determinations of the relative densities of hydrogen and oxygen, and of their combining volumes. From these data the weight of oxygen combining with unit weight of hydrogen can be calculated readily. Towards the close of the eighteenth century and early in the nineteenth century, several crude attempts were made to determine the atomic weight of hydrogen. In 1788 Monge, Lavoisier, and Meusnier found that 12 volumes of oxygen combine with 22.924 volumes of hydrogen, or in weight proportions 1 part of hydrogen with 6.61 parts of oxygen. The corresponding ratio according to the modern system is

H:O=1:13.22.

Three years later Fourcroy, Vauquelin, and Seguin observed during a prolonged experiment that 12570.942 cubic inches of oxygen and 26017.68 cubic inches of hydrogen, reduced to 14° C. and 28 inches of mercury, combined to form 7249 grains of water. By direct weighing, a cubic inch of water was found to weigh 0.4925 grain, and of hydrogen 0.040452 grain. The combined weights of the two gases exceeded the weight of the water formed by 0.277 grain. It follows from the results that 1 part by weight of hydrogen combines with 6.17 parts of oxygen, giving the ratio

H:O=1: 12.34.

In 1803 John Dalton fixed the atomic weight of hydrogen as unity, and gave the ratio of the atomic weights of hydrogen and oxygen acccording to the custom of the day as H:O=1:5.5, a value far wide of the mark. In view of the statements of Gay-Lussac and Humboldt that water was formed by the combination of 2 volumes of hydrogen with 1 volume of oxygen, and those of Cavendish and Lavoisier that oxygen was fourteen times as heavy as hydrogen, Dalton in 1808 substituted the ratio

H:O=1:7.

The first calculation with any approach to accuracy was that of Wollaston in 1814. From the combining volumes of hydrogen and oxygen as given by Gay-Lussac and Humboldt in 1805, coupled with Biot and Arago's determination of the densities of the two gases, Wollaston calculated the atomic weight of hydrogen to be 6.64 (O=100). Expressed in modern terms the ratio is

H:O=1:15.09=1.06:16.

Although the hypothesis of Avogadro and Ampere as to the relation between the densities and molecular weights of gases was propounded in 1811, half a century elapsed before its acceptance by the chemical world. Gay-Lussac's Law of Volumes, enunciated in 1808, led Berzelius to the assumption that the densities of elementary gases are proportional to their atomic weights. Acting on this assumption, he and Dulong in 1821 determined the densities of hydrogen and oxygen, the results being referred to air as unity. For hydrogen the density was found to be 0-0688, and for oxygen 1-1026, the ratio being

H:O=1:15.9538=0.9984:16.

This experiment gives as the atomic weight of hydrogen 0.9984 (O=16), a result in good accord with that obtained by the same investigators by gravimetric methods.

Subsequent research proved that Gay-Lussac's Law of Combining Volumes is not exact, but only an approximation, so that oxygen and hydrogen do not behave as perfect gases. For an accurate calculation of the relative values for the atomic weights of hydrogen and oxygen, it is therefore essential to know not only the relative densities of the two gases, but also the exact ratio of their combining volumes. It will be convenient to consider each of these problems separately, and subsequently to calculate the relative atomic weights of the two elements by combining the results.

(a) The Relative Densities of Hydrogen and Oxygen at N.T.P.

In 1841 Dumas and Boussingault gave for the density of hydrogen the mean value 0-0693, and for oxygen 1-1057 (air=l). The ratio of these densities is

DH:DO=l:15.9628=1.0029:16.

In his work on the density of gases, Regnault balanced the glass globe in which the gas was weighed against a second globe of similar capacity, and as nearly as possible of the same weight. The mean densities found were 0-069263 for hydrogen and 1-105633 for oxygen (air=l), the ratio of the densities being

DH:DO=l:15.9628=1.0023: 16.

A source of error overlooked by Regnault was first observed by Mendeleeff in 1875. When a glass globe is evacuated, the pressure of the atmosphere causes a slight contraction in volume, so that, when evacuated and weighed in air, it displaces a smaller quantity of air than when weighed full of hydrogen or any other gas. The weight of hydrogen determined by difference is accordingly too low. The discrepancy is small, but not negligible, for when the two gases differ greatly in density the error introduced by omitting the correction is very considerable.

The necessary correction to Regnault's work was made by Crafts, and yielded for the density of hydrogen the value 0.06949, and for oxygen 1-10562, the ratio of the densities being

DH:DO=l:15.9105=1.0056:16.

In 1888 Rayleigh weighed hydrogen and oxygen in the same glass globe and corrected for the reduction in the volume of the globe, the ratio of the two densities being

DH:DO=l:15.884=1.0073:16.

The mean of three determinations of the density of hydrogen made by Cooke is 0.06958 (air=1), and in conjunction with Crafts's value for the density of oxygen gives the ratio

DH:DO=l:15.890=1.0069:16.

In 1891 Leduc adopted Regnault's method, and made three determinations of the density of hydrogen, and the same number of the density of oxygen, the values obtained being 0.06948 and 1.10506. The ratio of these densities is

DH:DO=l:15.905=1.0060:16.

In 1892 Rayleigh employed hydrogen and oxygen prepared by electrolysis. As the mean of nineteen experiments with hydrogen and eleven with oxygen, he found the ratio of the densities to be

DH:DO=l:15.882=1.00743:16.

In 1893 he determined the densities of the two gases relative to air, the ratio being

DH:DO=l:15.8818=1.00744:16,

a result almost identical with that found by him in the preceding year.

In 1895 Morley published an account of his researches. The gravimetric results of his investigations have already been considered on p. 41; the physico-chemical sections are briefly summarized here. The weight of a litre of oxygen at 0° C. and 760 mm., reduced to sea-level in the latitude of 45°, was determined by three series of experiments involving nine, fifteen, and seventeen observations respectively, the mean value for each series being
  1. 1.42879 ±0.000051 grams.
  2. 1.42887 ±0.000048 grams.
  3. 1.42917 ±0.000048 grams.
The mean value of the three series is

1.42900 ±0.000034 grams.

The oxygen was prepared from potassium chlorate.

The weight of a litre of hydrogen at 0° C. and 760 mm., reduced to sea-level in the latitude of 45°, was determined by five series of experiments, the number of observations being fifteen, nineteen, eight, six, and eleven respectively. The value obtained for each series was -

  1. 0.089938 ±0.000007 gram.
  2. 0.089970 ±0.000011 gram.
  3. 0.089886 ±0.0000049 gram.
  4. 0.089880 ±0.0000088 gram.
  5. 0.089866±0.0000034 gram.


The mean value of all the series is 0.089897, but Morley considered the results of the first and second series to be too high, owing to the presence of mercury-vapour in the glass globe. This source of error was eliminated in the other three series, the mean value calculated from them being 0.089873 ±0.0000027 gram.

These results give for the mean ratio of the densities of the two gases

DH:DO=l:15.9002=1.00628:16.

In 1897 Thomsen employed the aluminium method, and determined the weight of a normal litre of hydrogen to be 0.089947±0.000012 gram, and the corresponding weight of oxygen, prepared from potassium chlorate, to be 1.42906 ±0.00004 grams. The ratio of the densities is

DH:DO=l:15.8878 =1.00730: 16.

For the sake of ready comparison, the foregoing results are grouped together in tabular form.

The Ratio of the Combining Volumes of Hydrogen and Oxygen at N.T.P.

In 1892 Leduc gave the ratio of the combining volumes of hydrogen and oxygen as

VH:VO=20037:1.

Morley has shown this result to be somewhat high on account of the neglect of certain errors.

In 1893 Scott found the value

VH:VO=2.00245:1

at the temperature of the laboratory, the ratio as recalculated for N.T.P. being

VH:VO =2.00285:1.

In 1893 Morley determined the ratio by a eudiometric method, but in 1895 he discarded the results obtained, when publishing an investigation of the ratio by another method. The weight of a normal litre of detonating gas, prepared at 0° C. by electrolysis of an aqueous solution of sodium hydroxide formed by dissolving the metal in water, was determined to be 0.535510 gram. From this number, and the corresponding values for hydrogen and oxygen, the ratio of the combining volumes of hydrogen and oxygen was calculated, allowance being made for the slight excess of hydrogen always present after the explosion of detonating gas prepared at 0° C. At N.T.P. the ratio found was

VH:VO =2.00269:1.

Employing another method, Rayleigh found the value VH:VO =2.0026:1.

In 1916 a very accurate determination of the ratio was made by Burt and Edgar. The hydrogen was prepared by the electrolysis of barium hydroxide, and the oxygen either by the same method or by heating potassium permanganate. The measurements were made at N.T.P., and a slight excess of hydrogen was employed. Five series of experiments gave the volume ratio

VH:VO=2.00288:1.

The Atomic Weight Ratio.

Of the foregoing data, the results of Morley and of Burt and Edgar are the most trustworthy. Combining Morley's ratio for the relative densities of the gases with that for their combining volumes, the atomic weight of hydrogen (O=16) is given by the expression

Atomic weight of hydrogen = 1.00628 × 2.00269 / 2 = 1.00763.

This result approximates closely to that obtained by Morley in his gravimetric researches.

If Morley's values for the weight of 1 litre of hydrogen (0.089873 gram) and of oxygen (1.42900 gram) are employed, Burt and Edgar's ratio gives for the atomic weight of hydrogen

1.00772.

Adopting Germann's more probable value for oxygen, 1.42905, the atomic weight of hydrogen becomes

1.00769,

which is probably the most accurate result hitherto obtained.

Other methods of calculation yield figures supporting those of Morley. They utilize the densities and other physical constants of hydrogen and oxygen, and are based on the purely physical methods of "limiting densities," "critical constants," and "molecular volumes."

Conclusion

Of the results recorded in the preceding account, the most important are -
  1. Morley's gravimetric result: H=1.00762.
  2. Morley's physico-chemical result: H=1.00763.
  3. Noyes's gravimetric result: H=1.00787.
  4. Burt and Edgar's physico-chemical result: H=1.00772.
The remarkably close similarity between the two results obtained by Morley is noteworthy. The fact that Burt and Edgar's value approximates very closely to the mean of the other three, H =1.00775, is strong evidence in favour of its accuracy. It is also worth while to point out that, if Morley's value for the atomic weight of hydrogen is accepted, the careful syntheses of hydrogen chloride effected by Edgar, and the volumetric analyses of the same compound by Gray and Burt, yield values for the atomic weight of chlorine in good agreement with the results of modern gravimetric analysis. If Noyes's value for hydrogen is adopted in the calculations, high values for chlorine are obtained.

In the opinion of Noyes, the most trustworthy value is the mean between his own and Morley's gravimetric result, H=1.00774, and this view is strongly supported by the work of Burt and Edgar, and by calculations made by the method of limiting densities.

In 1898 the German Committee on Atomic Weights selected 1.01 as the atomic weight of hydrogen. In 1903 the International Committee on Atomic Weights altered the number to

H =1.008,

a value still recognized at the present time. In this series of textbooks the value

H = 1.00762

has been selected for the calculation of atomic weights.
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