M. V. Regnault on the Expansion of Gases. 141 



ture and elastic force in the various parts of a current of gas when 

 it undergoes sudden or successive modifications of its density, 

 we should have a simple means of determining the quantities of 

 heat which are lost or gained in the various parts of its course. 

 My memoir contains a great number of experiments made in 

 this direction. 



I have employed in succession mercurial thermometers, air- 

 thermometers, and thermoelectric elements. The expansion 

 sometimes took place through an orifice in a thin plate, some- 

 times through a capillary tube^ and sometimes through porous 

 plates. It is impossible to give an account of these experiments 

 within the limits to which I must confine this extract. 



A large reservoir contains air under a pressure of 8 metres of 

 mercury which is maintained virtually constant. It is enclosed, 

 as usual, in a large vessel of water, the temperature of which re- 

 mains unchanged during an experiment. The reservoir com- 

 municates with a worm placed in the same vessel; its length is 

 12 metres, and internal section 20 millims. 



The end of the worm just projects from the side of the large 

 vessel, and is closed by a metal plate in which is perforated a 

 small orifice. A tube of thin brass, the length of which can 

 be varied, fits, with some friction, on the projecting part of the 

 worm. 



The temperature of the air after its escape through the capil- 

 lary orifice is determined by a mercurial thermometer, the bulb 

 of which can be brought more or less near the orifice. 



When the bulb of the small thermometer is at about 10 mil- 

 lims. from the capillary orifice, it marks 1° # 8 less than the ther- 

 mometer of the large vessel. If it be gradually removed from 

 the orifice, the temperature rises rather rapidly, and the differ- 

 ence of temperature is only some tenths of a degree when the 

 bulb of the small thermometer approaches the orifice of the 

 large terminal tube. 



If from the distance of 10 millims., at which there was a dif- 

 ference in temperature of 1 0, 8, the bulb of the small thermo- 

 meter is gradually brought nearer the orifice, the difference of 

 temperature, instead of increasing, diminishes very briskly; it 

 even changes in sign when the bulb of the thermometer is 

 brought almost into contact with the sides of the orifice. 



Thus in this experiment the small thermometer shows the 

 greatest cooling when its bulb is 10 millims. from the orifice. 

 The cooling diminishes as the bulb is removed from the capil- 

 lary orifice, because the velocity of the gas diminishes ; it also di- 

 minishes, and that very rapidly, when the distance is made less 

 than 10 millims. And the cooling changes into an apparent 

 heating when the bulb almost touches the orifice. 



