354 Dr. Thomson's Answer to Mr, Rainy, [Nov. 



water of tlie temperature 49® during the whole time that the zinc 

 was dissolving, m order tb prevent aiiy elevation of temperature. 



When the zinc was completely dissolved, the flask was taken 

 out of the water trough, and wiped dry on the outside. It was 

 then laid upon a t^ble with its mouth open, and gently moved 

 for about ten niiriutes to allow the two cubic ihches of hydrogen 

 gas which it contained to make their escape, and common air to 

 take its place. It was then weighed, taking care to allow it to 

 remain on the scales till the weight became stationary, if it was 

 not so at first, which, however, was generally the case. 



When the tube containing the chloride of calcium was 

 detached from the flask, I put that extremity of it which had 

 been furthest from the flask into my mouth, and, drawing a long 

 breath, displaced the whole of the hydrogen gas which it con- 

 tained, substituting in its place the common air of the room. 

 The tube was then wiped dry and weighed. 



After these details, which will enable the reader to appreciate 

 the degree of confidence which may be put in the experiments, 

 I shall follow Mr. Rainy through his calculations. 



Mr. Kainy says that I have employed an erroneous formula 

 in calculating the specific gravity of vapour. In fact, however, 

 tlie formula which 1 used is precisely the same as his ; excepting 



that he has introduced an additional term —. I didi]ipf;cf|ij]^ffler 



it as worth while to introduce this additional term ; because 

 some uncertainty still hangs over the value of p, and must con- 

 tinue to do so till the law of the expansion of vapour be accu- 

 rately determined. But I have no objection, since Mr. Rainy 



chooses it, to introduce the term ^, It will in fact produce only 



a very trifling alteration in the result. 



I must begin by reminding Mr. Rainy that the boiling point 

 of the liquid from which the hydrogen gas was evolved was not 

 212°, but 224^, or 12° above the boiling point of water. It was 

 a liquid that required to be raised 12° higher than water before 

 it gave out the same quantity of vapour that water placed in the 

 same circumstances would do. It was necessary on this 

 account to reduce the temperature of 49° (at which the experi- 

 mefit was made) by 12° ; for the vapour given out was what 

 would have been given out by water of the temperature 37°. 



The volume of gas extricated at 49° was 137*08 cubic inches. 

 To find its volume at 60°, we have this analogy, 497 : 508 :: 

 137-08': 140-396 cubic inches = volume of gas at 60°. 



The pressure of vapour at 37° (according to Dalton's table) is 

 0*237 inch of mercury =r p. Let us calculate the specific gravity 



of this vapour by Mr Rainy's formula, which isr^,- x |^ x 0*625 

 = Rpecific gravity^ 



