192 Mr. Rainyh Reply to Dr. Thomson. [Makch, 



himself out of this difficulty by assmnhig that the atom of azote 

 is to that of oxygen as 1'75 to 1, and then infers that the specific 



gravity of azote is 0'972. 



3. " The specific gravity of ammoniacal gas deduced from a 

 mean of Sir H. Davy's experiments and Dr. Thomson's is 

 0'690237. It has been satisfactorily proved that it consists of 

 1 volume azote and 3 volumes hydrogen condensed into 2 vo- 

 lumes. Hence if we admit that the specific gravity of azote is 



0'972, it will follow that the specific gravity of hydrogen is 

 0"069417." This evidence appears at first sight satisfactory ; but 

 if we assume Dr. Thomson's experiments on ammoniacal gas, as 

 the basis of calculation, we shall find the specific gravity of 

 hydrogen to be 0*071, or 0*073 if we take 0'59669, the specific 

 gravity of ammonia assigned by Biot and Arago. I see no 

 reason for giving a preference to Sir H. Davy's result in this 

 case, except that it coincides more closely with the hypothesis. 



4. Adopting the specific gravities of oxygen and hydrogen, 

 as determined in the last two paragraphs, it will follow that the 

 atom of hydrogen is to that of oxygen as 1 to 8*003, a coinci- 

 dence so near that we may safely assume the real ratio to be 

 1 to 8. This of course I admit, but the accuracy of the conclu- 

 sion rests entirely on the correctness of the premises, which I 

 have already called in question. 



5. " The specific gravity of vapour upon which Mr. Rainy lays 

 so much stress, is founded on the assumption of this ratio. The 

 specific gravity of vapour has been settled at 0*625. Now 

 vapour is a compound of 1 volume of hydrogen and i volume of 

 oxygen united together, and condensed into 1 volume. If we 



subtract 0*555 from 0*625, the remainder 0*0694 must represent 

 the specific gravity of hydrogen gas, and 0*0694 : I'llll :: 1 : 

 16. Therefore all the calculations and objections of Mr. Rainy 

 •were founded on the admission of the very ratio which he endea- 

 vours in his paper to overturn." 



I have no doubt that the specific gravity of vapour is nearly 

 0*625 or0*624, as ascertained by Gay-Lussac. I did not assume 

 0*625 as mathematically correct, but partly because I did not 

 wish to diflPer with Dr. Thomson on a point which had no mate- 

 rial bearing on the argument. It was obvious to me that the 

 number 0*625 was not affected with any error that could at all 

 interfere with my conclusions. Supposing the specific gravity 

 of vapour to be as low as 0*6, or as high as 0*65, the objections 

 which I have urged still seem to me unanswered. But admitting 

 that the specific gravity of vapour is rigidly 0*625, and that each 

 volume of it contains 1 volume of hydrogen and half a volume of 

 oxygen, I deny that from these data any inference can be made 

 regarding the ratios of the specific gravities of oxygen and 



