OF GASES AND VAPOURS. 191 
by heat, when a beautiful confirmation results with respect to 
steam, for the densities of the aqueous vapours with the proper 
coefficient of expansion turn out exactly as Gay-Lussac, Schmed- 
dingk and Munke found them, for different temperatures. 
The formula given, moreover, serves to determine the specific 
heats of vapours, respecting which we had previously scarcely 
any information. 
At the conclusion I give some other formule for calculating 
the effect of steam-engines founded on the preceding, and I show 
how great an amount of the motive force of the heat in steam- 
engines is lost from its being everywhere requisite to overcome 
the cohesion of the water in the formation of steam. The formulz 
here given are purely theoretical ; but they are the fundamental 
basis of a practical theory of the steam-engine corresponding to 
the present state of science. 
Manheim, Aug. 1844. C. HoLttzMann. 
§ 1. Determination of the Quantity of Heat. 
If heat be added to a quantity of gas which I imagine to be in 
a vessel impermeable to heat, the phenomena resulting are, in- 
crease of temperature, of elasticity and of volume; and there 
may occur,— ; 
a. Increase of temperature and of elasticity without any 
change in the volume ; and 
6. Increase of temperature with change of volume, but with- 
out any alteration of the elasticity. 
In the first case no motion of matter occurs; in the second 
case, on the contrary, the effect of the heat is in part increase 
of temperature and in part a motion of matter. But the effect 
of the heat may likewise be simply represented by a generated 
motion. For this purpose it is only requisite in both the cases 
considered to allow the gas to expand without alteration of its 
quantity of heat, until it has again acquired the previous tem- 
perature. In this case the effect of the heat added consists 
solely in this expansion, and consequently the pressure under 
which the gas stands is driven back through a certain space, 
which in all cases must be regarded as equivalent to the raising of 
a weight to a height, so that in this case the effect of the heat 
consists solely in a mechanical action, and is measured by it. 
P2 
