March 14, 1^89] 



NA TURd 



463 



equivalent quantities of the two gases. The oxygen was 

 first admitted until the level of the mercury had dropped 

 to a certain mark, and subsequently the hydrogen down 

 to a second mark, whose position relatively to the first 

 was determined by preliminary measurements of volume. 

 The mixed gases might then be drawn off into the eudio- 

 meter until exhausted, after which the chamber might 

 be re-charged as before. But a good deal of time may 

 be saved by replenishing the chamber from the globes 

 simultaneously with the exhaustion into the eudiometer. 

 In order to do this without losing the proper proportion, 

 simple mercury manometers were provided for indicating 

 the pressures of the gases at any time remaining in the 

 globes. But even with this assistance close attention was 

 necessary to obviate an accumulation of residual gas in 

 the eudiometer, such as would endanger the success of 

 the experiment, or, at least, entail tedious delay. To ob- 

 tain a reasonable control, two sparking places were pro- 

 vided, of which the upper was situate nearly at the top of 

 the eudiometer. This was employed at the close, and 

 whenever in the course of the combustion the residual 

 gas chanced to be much reduced in quantity ; but, as a 

 rule, the explosions were made from the lower sparking 

 point. The most convenient state of things was attained 

 when the tube contained excess of oxygen down to a 

 point somewhat below the lower sparking wires. Under 

 these circumstances, each bubble of explosive gas readily 

 found its way to the sparks, and there was no tendency 

 to a dangerous accumulation of mixed gas before an 

 explosion took place. When the gas in excess was 

 hydrogen, the manipulation was more difficult, on account 

 of the greater density of the explosive gas retarding its 

 travel to the necessary height. 



In spite of all precautions several attempted determina- 

 tions have failed from various causes, such as fracture of 

 the eudiometer and others which it is not necessary here 

 to particularize, leading to the loss of much labour. Five 

 results only can at present be reported, and are as 

 follows : — 



December 24, 1888 1593 



January 3, 1889 15 98 



21, „ 15-98 



February 2, ,, ... .. I5'93 

 13. 1592 



Mean 15 '95 



This number represents the atomic ratio of oxygen and 

 hydrogen as deduced immediately from the weighings 

 with allowance for the unburnt residue. It is subject to 

 the correction for buoyancy rendered necessary by the 

 shrinkage of the external volume of the globes when in- 

 ternally exhausted, as explained in my former communi- 

 cation.^ In these experiments, the globe which contained 

 the hydrogen was the same (14) as that employed for the 

 ■density determinations. The necessary correction is thus 

 four parts in a thousand, reducing the final number for 

 the atomic weight of oxygen to — 



15-89, 

 somewhat lower than that which I formerly obtained 

 {i5'9i) by the use of Mr. Scott's value of the volume ratio. 

 It may be convenient to recall that the corresponding 

 number obtained by Cooke and Richards (corrected for 

 shrinkage) is IS'S/, while that of Keiser is I5'95. 



In the present incomplete state of the investigation, I 

 ■do not wish to lay much stress upon the above number, 

 more especially as the agreement of the several results is 

 not so good as it should be. The principal source of 



' The necessity of ih's c rrection wa'? recogniz-d .it an early stage, and. if 

 T remember rightly, was one of the reasons which led me to think that a re- 

 •determinalion of the den iiy of hydrogen was desirable. In the meantime, 

 however, the question was discussed by .\gamennone {Atti {Rendiconii) d. 

 Jf. Accad. dei Lincei, 18S5), and s->me noiice of his work reached me. 

 When writ'ng my paper last year I could not recall the circumstances ; but 



I^ince the matter has attracted attention I hive made inquiry, and take this 

 -opportunity of pointing out that the credit of first publication is due tj 



error, of a non-chemical character, is in the estimation of 

 the weight of the hydrogen. Although this part of the 

 work cannot be conducted under quite such favourable 

 conditions as in the case of a density determination, the 

 error in the difference of the two weighings should not 

 exceed o'ooo2 gramme. The whole weight of the hydrogen 

 used is about 01 gramme ; ' So that the error should not 

 exceed 3 in the last figure of the final number. It is 

 thus scarcely possible to explain the variations among the 

 five numbers as due merely to errors of the weighings.. 



The following are the details of the determination of 

 February 2, chosen at random : — 



Before combustion ... G14+ II +0'29d6=:Gji ... pointer 2005 

 After ,, ... G,4+H +0-4006 = Gji ... pointer 20*31 



Hydrogen taken = oiioo-ooodo-«»o-i 0995 gramme. 



Before combustion ... Gj3 + = G,j + 2"237 ... pointer 2000 



After ,, ... G,3 + = Gii-f-i-357 ... pointtri9-3 



Oxygen taken =o88oo + o"OJOl:;;o 8801 gramme. 



At the close of operations the residue in the eudio- 

 meter was oxygen, occupying 7-8 cubic centimetres. 

 This was at a total pressure of 296 - 16 2 = 134 inches 

 of mercury. Subtracting 0*4 inch for the pressure of 

 the water vapour, we get 130 as representing the oxygen 

 pressure. The temperature was about 12^ C. Thus, 

 taking the weight of a cubi'^ centimetre of oxygen at 

 o" C, and under a pressure of 76'o centimetres of 

 mercury to be 000143 gramme, we get as the weight 

 of the residual oxygen — 



7'8 I'?'OX2'^4 ^ 



o"ooi43 ' ^J — ='^ = 00046 gramme. 



^^ 1 + 12x000367 760 ^ ^ 



The weight of oxygen burnt was, therefore, o'SSoi - 

 o'oo46 = 0*8755 gramme. 



Finally, for the rate of atomic weight — 



^'^y^^" =i5-926. 

 i Hydrogt n 



In several cases the residual gas was subjected to 

 analysis. Thus, after the determination of February 2, 

 the volume was reduced by additions of hydrogen to 

 12 cubic centimetre. On introduction of potash there 

 was shrinkage to about 09, and, on addition of pyro- 

 gallic acid, to o;r or o"2. These volumes of gas are 

 here measured at a pressure of \ atmosphere, and are, 

 therefore, to be divided by 3 if we wish to estimate the 

 quantities of gas under standard conditions. The final 

 residue of (say) o'05 cubic centimetre should be nitrogen, 

 and, even if originally mixed with the hydrogen— the 

 most unfavourable case — would involve an error of only 

 .,j^,-,g in the final result. The 01 cubic centimetre of car- 

 bonic anhydride, if originally contained in the hydrogen, 

 would be more important, but this is very improbable. 

 If originally mixed with the oxygen, or due to leakage 

 through india-rubber into the combustion apparatus, it 

 would lead to no appreciable error. 



The aggregate impurity of 015, here indicated, is toler- 

 ably satisfactory in comparison with the total quantity of 

 gas dealt with— 2000 cubic centimetres. It is possible, 

 however, that nitrogen might be oxidized, and thus not 

 manifest itself under the above tests. In another experi- 

 ment the water of combustion was examined for acidity, 

 but without definite indications of nitric acid. The 

 slight reddening observed appeared to be rather that 

 due to carbonic acH, some of which, it must be remem- 

 bered, would be dissolved in the water. These and 

 other matters demand further attention. 



The somewhat complicated glass-blowing required for 

 the combustion apparatus has all been done at home by 

 my assistant, Mr. Gordon, on whom has also fallen most 

 of the rather tedious work connected with the evacuation 

 of globes and other apparatus, and with the preparation 

 of the gases. 



' It was usual to take for combustion from two-thirds to thr:e-fjurlhs of 

 the contents of the globe. 



