30 
BRITISH ASSOCIATION.— 1835 . 
CHEMISTRY.—ELECTRICITY. 
On the Specific Heats of the permanently elastic Fluids. By 
James Apjohn, M.D., Professor of Chemistry in the Royal Col- 
lege of Surgeons, Ireland. 
After an introductory view of the state of knowledge on this sub¬ 
ject, Dr. Apjohn proceeded to explain the principle of an entirely 
new method which he was enabled to apply to the investigation of 
the difficult problem under consideration, in consequence of having 
been recently fortunate enough to arrive at a formula which ex¬ 
presses, with extreme and unexpected precision, the relation exist¬ 
ing between the indications of a wet-bulb thermometer and the cor¬ 
responding dew-points. This formula* (see Proceedings of Ma¬ 
thematical and Physical Section, p. 27,) being equally true of all 
gases, obviously suggests a method of comparing their specific 
heats. For as in the case of every gas it may be deduced that 
(f'-f")e SO . 
a — J ‘ X —, it is clear that if we determine values of /', f n 
48 dp 
and d in different elastic media, we shall have data for ascertaining 
their relative capacities for caloric. Such a method, however, though 
theoretically exact, is beset with difficulties so great that it may be 
considered as practically impossible. The artificial gases, as usually 
collected, are saturated with moisture, a state in which they are 
quite unsuited for the necessary experiments; and even though this 
difficulty were overcome, it would probably be impossible to deter¬ 
mine their dew-points by direct experiment. 
If, however, we suppose that/'^O, or that the gas is perfectly 
dry, the above value of a will become — 7 x —, an expression in- 
48 d p 
volving no unknown quantities but f and d, and which will there¬ 
fore enable us to calculate the specific heat of a gas when we have 
observed the stationary temperature t', to which, when in a state of 
perfect desiccation, it brings the wet-bulb thermometer. In order 
to the determination of t' , and of t — t'=d, the following method of 
experimenting was, after a trial of several others, finally adopted. 
Into a bent tube, abode, about 50 inches long, and-i^ths of an 
inch in diameter, oil of vitriol was poured to the height marked by 
the horizontal line x y, and to one extremity of this siphon a pair 
of bladders furnished with stopcocks were attached, through the 
intervention of a three-armed copper pipe, while to the other ex¬ 
tremity of the apparatus there was connected by a caoutchouc collar 
glass tube carrying the dry thermometer D, and wet one W. 
* f"=f — X in which f" is the elastic force of vapour at the 
dew-point; f its elastic force at the temperature t } , shown by the wet ther¬ 
mometer ; d the difference between the latter temperature, and t that of the 
air; a the specific heat of air; and e the caloric of elasticity of the vapour 
of water whose elastic force or tension is represented by /'. 
