1822.] Dr. Apjohn on the Specific Gravity of Gases. 195 
ArTICLE VI. 
Additional Remarks on the Influence of Moisture in modifying 
the Specific Gravity of Gases. By John Apjohn, MD. 
(To the Editor of the Annals of Philosophy.) 
SIR, Trinity College. 
in the number of the Annals for May last, you did me the 
favour of publishing some observations of mine upon the influ- 
ence of moisture in modifying the specific gravities of the gases. 
In that paper, I gave an expression for the specific gravity of a 
gas saturated with moisture, and also suggested a method of 
determining the exact specific gravity of a gas perfectly dry. 
The principle upon which I proceeded, namely, that the density 
of steam is directly as its tension, has been called in question 
by Mr. Herapath, in a succeeding number of your journal, in a 
paper, in which he also contests the correctness of Dr. Thom- 
son’s idea of the sensible and latent heat of steam, beginning at 
32, constituting a constant quantity. With this latter topic, I 
have no concern. But as Mr. H. conceives that he has proved 
from received principles that the density of steam is not simply 
as its tension, and as he appears to me to have by no means 
accomplished what he asserts, I shall briefly state the reasons 
which have led me to this opinion. Mr. H. has certainly 
adopted the most decisive method for achieving his object, for 
he proceeds at once to show what the true relation between 
them is. I hope, however, to prove, that the gentleman has 
fallen into an error, and that this errer consists in his confound- 
ing gases and vapours, substances, as to many even of their phy- 
sical properties, essentially distinct, and as to none more so than 
the relation existing between the density, temperature, and elas- 
ticity of each. Before_I proceed, I beg to be understood as 
admitting that the expression S’ =S8. en a sie gi 
H. properly represents the relation between the density and 
tension of a permanently elastic gas, or even of steam when 
separated from the water which has produced it. That the fol- 
lowing remarks may be the better understood, I shall give the 
steps which lead to this expression. Gases, and even steam 
isolated, have been found by experiment to expand the ;4,th of 
their volume for every degree of Fahrenheit. Hence it follows, 
that if 480 represent the tension of any of them at 32, the ten- 
sion, at any higher temperature F, will be 448+F. The follow- 
ing proportion then may be instituted. 448 + F : 448 + ¥ re 
7 448 + F" » , iidlaedetiadie 
aE é again, since 
METH the Mie at F’. Andagat yA 
oO ‘ 
ven by Mr. 
7 the tension at F; 
