138 M. V. Kegnault on the Expansion of Gases. 



It will be seen that the cooling increases more rapidly than 

 the excess of pressure which produces the outflow. 



The considerable cooling which carbonic acid undergoes by its 

 expansion when it retains its acquired vis viva, explains several 

 phenomena which have been observed on carbonic acid liquefied 

 in Thilorier's apparatus. Thus, when the liquid acid escapes 

 from the reservoir through a narrow orifice, and is received in a 

 small open receiver, a great part of the liquid congeals to a snowy 

 mass which fills the whole of the receiver. This effect is ordi- 

 narily attributed to the heat absorbed by carbonic acid when it 

 assumes the gaseous state ; this heat would be imparted to it 

 by the carbonic acid which remains liquid, the temperature of 

 which thus sinks below its freezing-point. If this explanation 

 were correct, the solid acid should present the appearance of a 

 continuous mass applied against the sides of the receiver, and 

 not as a snowy mass which completely fills it. The evapora- 

 tion of the liquid certainly gives a gas whose temperature is very 

 low ; but the subsequent expansion of this gas greatly depresses 

 its temperature again ; so that it is really the gaseous carbonic 

 acid formed which congeals, and not that which remains liquid. 



When the gaseous acid which forms the atmosphere above the 

 liquid acid is allowed to escape from the reservoir by a capillary 

 orifice, it is observed that the orifice is frequently closed by small 

 crystals of solid acid. To the cold produced by the expansion 

 is due this momentary congelation. 



In fine, my experiments with capillary silver tubes prove that 

 when a gas flows, even with considerable velocity, along very 

 extended sides, there is no appreciable disengagement of heat 

 which could be attributed to the friction of the gaseous mole- 

 cules against these sides. 



This conclusion is opposed to the view generally held ; and 

 many facts may be cited which seem to contradict it. I will 

 mention the most important. 



A projectile which traverses the air with great velocity becomes 

 much heated. This fact is attributed to the heat disengaged 

 by the friction of the projectile against the molecules of the air 

 which it traverses. 



Meteorites fall through our atmosphere with extreme velo- 

 city; they become heated to incandescece, and melt, either en- 

 tirely or merely on their surface. This fact is attributed to the 

 heat disengaged by friction against the gaseous molecules. 



I think that in both cases the heat comes from another cause, 

 and is merely due to the heat disengaged by the compression of 

 the air. 



When a body in motion traverses the air with a greater velo- 



