260 Mr. Sylvester on the Specific Gravity of Gases. [Ocr. 
ARTICLE V. 
Additional Remarks on Dr. Thomson’s Paper on. the Effect of 
Aqueous Vapour on the Specific Gravity of Gases. By Charles 
Sylvester, Esq. 
(To the Editor of the Annals of Philosophy.) 
DEAR SIR, Carnarvon, Aug. 15, 1822, 
In my letter to you which appeared in the Annals for July, 
relative to Dr. Thomson’s paper on the subject of high pressure 
steam, and the influence of aqueous vapour on the specific gra- 
vity of gases, my remarks were principally directed to Dr. 
Thomson’s observations upon high pressure steam. I also made 
some allusion to his formula for correcting the specific gravity of 
gases from the pressure of aqueous vapour. Having since had 
a conversation with Mr. Dalton, of Manchester, | have been 
induced to attend more minutely to that part of Dr. Thomson’s 
paper, and find that his formula will only be correct when the 
experiment is made at 212° for aqueous vapour, or the boiling 
point of the substance from which the vapour is derived. At any 
temperature above or below that point, the formula given by Dr. 
Thomson will fail. My object in this communication is to give 
a formula which, I believe, will give the true result at all.tem- 
peratures. 
Let P = a column of mercury equal to the pressure of the 
atmosphere ; f = a column of mercury which a volume of any 
condensible vapour will support, unmixed with any other elastic 
fluid, and at any given temperature ; S = the specific gravity of 
the gas under examination; c = the specific gravity of the 
vapour under ‘the pressure P; and R = the resulting specific 
gravity. Then if we suppose the gas and the vapour to be in 
separate equal.volumes, and at the temperature of the boiling 
point of the liquid producing the vapour, these volumes after 
mixture will be 1 + 1 = 2, the general expression being 1 + 
£ which when f = P, will be 1 + 1 = 2. From the expression 
f 
v the resulting volume, we have the following equation 
to find R the resulting specific gravity (1 + ) R=S+4+ 
i, and R = =< —_ WED ccc ey se = ©, each of the yo- 
lumes equal 1. I am, yours igre 
. SYLVESTER. 
