130 M.V. Regnault on the Expansion of Gases, 



Second case. — The calorimetrical reservoir contains gas at 

 rest under the pressure of the external atmosphere. The gas 

 compressed in a large reservoir escapes by a capillary orifice and 

 passes at a known temperature into the calorimetrical reservoir, 

 the gas of which it successively compresses until the elastic force 

 has become the same in the two reservoirs. 



Dynamical Expansion. — First Part. 



I. Heat absorbed by the expansion of a compressed gas when it 

 traverses a small orifice in a thin plate. (Experiments of 1850 

 to 1854.) 



The gas is compressed in a large reservoir ; it traverses a long 

 serpentine which keeps it at a constant temperature as far as its 

 capillary orifice, through which it expands and becomes in equi- 

 librium with the external pressure. The ehange of temperature 

 which it thus undergoes is determined by the calorimetrical me- 

 thod. I have discriminated two cases. 



First case. — The compressed gas reaches the capillary orifice 

 with a temperature but little different from that of the calori- 

 meter. In this case — 



Atmospheric air undergoes a diminution of temperature, which 

 amounts to o *3176 for a diminution in elastic force of 1 metre 

 of mercury. 



In the case of hydrogen, the variation in temperature is im- 

 perceptible. 



Lastly, for carbonic acid, the lowering of temperature is more 

 considerable than for atmospheric air; for it reaches 1°*64 for a 

 diminution in pressure of 1 metre of mercury. 



These effects are due to several causes acting simultaneously. 



1. Gases which deviate from Mariotte's law in the direction 

 ascertained for air and carbonic acid undergo an excess of expan- 

 sion which gives rise to a lowering of temperature proportional 

 to this excess ; as hydrogen deviates from this law in the oppo- 

 site direction, it probably disengages a small quantity of heat. 



2. The gaseous molecules are at rest in the large reservoir ; 

 on passing through the capillary orifice, not only do they expand 

 and drive back the external atmosphere, consuming the work ex- 

 pended by the tranquil gas in the reservoir, but they assume 

 successively a great velocity of translation, and therefore a vis 

 viva which is produced by their own heat, and which determines 

 a lowering of temperature which is frequently greater than that 

 due to the first cause. 



3. In proportion as the gas expands in the calorimeter the ve- 

 locity of translation of the molecules diminishes, and there is a 

 disengagement of heat which partially compensates for the cool- 

 ing due to the second cause; but there cannot be exact compen- 



