420 



NATURE 



[March i, 1888 



plete establishment of equilibrium before the tap of the 

 globe was turned off. Experiments on oxygen appeared 

 to show that two minutes was sufficient. For mea- 

 suring the atmospheric pressure, huo standard mercury 

 barometers were employed. 



The evacuations were effected by the Toppler to at 

 least 1/20000, so that the residual gas (at any rate after 

 one filling with hydrogen) could be neglected. 



I will now give some examples of actual i-esults. Those 

 in the following tables relate to gas prepared from sul- 

 phuric acid, with subsequent purification, as already 

 described : — 



Globe {l^),full. 



The second column shows that globe (14) and certain 

 platinum weights were suspended from the left end of ihe 

 beam, and the third column that (in this series) only the 

 counterpoising globe (11) was hung from the right end. 

 The fourth column gives the mean balance reading in divi- 

 sions of the scale, each of which (at the time of the above 

 experiments) represented 0-000187 gramme. The degree 

 of agreement of these numbers in the first part of the 

 table gives an idea of the errors due to the balance, and 

 to uncertainties in the condition of the exteriors of the 

 globes. A minute and unsystematic correction depend- 

 ing upon imperfect compensation of volumes (to the 

 extent of about 2 cubic centimetres) need not here be 

 regarded. 



The weight of the hydrogen at each filling is deduced, 

 whenever possible, by comparison of the "full" reading 

 with the mean of the immediately preceding and follow- 

 ing " empty " readings. The difference, interpreted in 

 grammes, is taken provisionally as the weight of the gas. 

 Thus, for the filhng of Nov. 5 — 



H = 0-154 - 2-25 X 0-000187 = 0-15358. 



The weights thus obtained depend of course upon the 

 temperature and pressure at the time of filling. Reduced 

 to correspond with a temperature of 12°, and to a baro- 

 metric height of 30 inches (but without a minute correc- 

 tion for varying temperature of the mercury) they stand 

 thus — 



November 5 ... ... ... 0-15811 



,, 8 015807 



,, 10 0-15798 



M 1-2 0-15792 



Mean 



0-15802 



so light, the mean of two accordant series being 

 0-15812. 



The weighing of oxygen is of course a much easier 

 operation than in the case of hydrogen. The gas was 

 prepared from chlorate of potash, and from a mixture of 

 the chlorates of potash and soda. The discrepancies 

 between the individual weighings were no more than 

 might fairly be attributed to thermometric and mano- 

 metric errors. The result reduced so as to correspond 

 in all respects with the numbers for hydrogen is 2-5186.^ 



But before these numbers can be compared, with the 

 object of obtaining the relative densities, a correction of 

 some importance is required, which appears to have been 

 overlooked by Prof. Cooke, as it was by Regnault. The 

 weight of the gas is not to be found by merely taking the 

 difference of the full and empty weighings, unless indeed 

 the weighings are conducted in vacuo. The external 

 volume of the globe is larger when it is full than when it 

 is empty, and the weight of the air corresponding to this 

 difference of volume must be added to the apparent weight 

 of the gas. 



By filling the globe with carefully boiled water, it is not 

 difficult to determine experimentally the expansion per 

 atmosphere. In the case of globe (14) it appears that 

 under normal atmospheric conditions the quantity to be 

 added to the apparent weights of the hydrogen and oxygen 

 is o 00056 gramme. 



The actually observed alteration of volume (regard 

 being had to the compressibility of water) agrees very 

 nearly with an a priori estimate, founded upon the theory 

 of thin spherical elastic shells and the known properties 

 of glass. The proportional value of the required correc- 

 tion, in my case about 4/1000 of the weight of the hydrogen, 

 will be for spherical globes proportional to «//, where a is 

 the radius of the globe, and t the thickness of the shell, or 

 to V/W, if V be the contents, and W the weight of the 

 glass. This ratio is nearly the same for Prof. Cooke's 

 globe and for mine ; but the much greater departure of 

 his globe from the spherical form may increase the amount 

 of the correction which ought to be introduced. 



In the estimates now to be given, which must be re- 

 garded as provisional, the apparent weight of the hydrogen 

 is taken at 0-15804, so that the real weight is 0-15860. 

 The weight of the same volume of oxygen under the same 

 conditions is 2-5 1 86 -}- 0-0006 = 2-5 192. The ratio of these 

 numbers is I5'884. 



The ratio of densities found by Regnault was 15-964, 

 but the greater part of the difference may well be ac- 

 counted for by the omission of the correction just now 

 considered. 



In order to interpret our result as a ratio of atomic 

 weights, we need to know accurately the ratio of atomic 

 volumes. The number given as most probable by Mr. 

 Scott, in May 1887,'^ was 1-994, but he informs me that 

 more recent experiments under improved conditions give 

 1-9965. Combining this with the ratio of densities, we 

 obtain as the ratio of atomic weights — 



2 X 15884 

 1-9965 



15-912. 



It is not improbable that experiments conducted on the 

 same lines, but with still greater precautions, may raise 

 the final number by one or even two thousandths of its 

 value. 



The ratio obtained by Prof. Cooke is 15-953 ; but the 

 difference between this number and that above obtained 

 may be more than accounted for, if I am right in my 

 suggestion that his gas weighings require correction for 

 the diminished buoyancy of the globe when the internal 

 pressure is removed. 



The hydrogen obtained hitherto with similar apparatus 

 and purifying tubes from hydrochloric acid is not quite 



' An examination of the weights revealed 

 account at present. 

 '^ Loc cit. 



no error worth taking into 



