282 
Specific heat of gases. 
Elementary 
principles. 
The instru- 
ment was a 
worm pipe 
and cooler ; 
to be heated 
by a current 
they give out in returning from the higher to the lower of 
those temperatures. We did not seek to determine the influ- 
ence which is exerted upon the phenomenon by the change of 
specific heat, determined in the gases by their dilatation cr con- 
traction, which take place by digression or elevation of tempe- 
rature j a change, which their great dilatability must render 
more sensible than in other bodies, and of which M. Gay 
Lussac has proved the existence. This department of enquiry 
could not have been made with our apparatus but with extreme 
difficulty. 
The means hitherto employed for the determination we 
sought, having all appeared liable to some objections, we 
determinedfto employ another, founded on the following consi- 
derations : 
Let us suppose a constant and uniform source of heat, of 
which the action shall be totally confined to the body A insu- 
lated in the air then this body will become gradually heated 
to a point at which, on account of its elevation of tempera- 
ture, above the surrounding air, it will lose as much as it 
receives. At this point the temperature will become stationary, 
if the temperature of the air continues unvaried. On the other 
hand, it is a principle generally admitted, and of which the 
truth cannot be disputed, particularly when small differences 
of temperature are concerned, that the differences of heat lost 
at each instant by a body insulated in the air, are proportional 
to the excess of its temperature above that of the surrounding 
air. Now, since the body A, when once arrived at its maxi- 
mum, loses as much heat as it receives j and that which it loses 
in a given time is proportional to the excess of its temperature 
beyond that of the surrounding air $ it must be concluded, 
that when arrived at this point, the quantity of caloric which is 
communicated to it by the source of heat, in a given time, is 
likewise proportional to the same excess. 
Let us now make a cylinder of thin copper 15 centimeters 
(5‘89 inches) in height, and 8 (3'14 inches) in diameter, filled 
with distilled water, and transversed by a worm pipe of more 
than 1 matre (39*3 inches) in length, forming a spiral of 8 
turns. 
