M. V. Regnault on the Expansion of Gases. 143 



worms formed of silver capillary tubes, where I could never ob- 

 serve any beat disengaged by friction. 



In my experiments on the flow of air through an orifice in a 

 thin plate, I observed that the thermometer indicated an increase 

 of temperature when its bulb was placed almost in contact with 

 the capillary orifice ; but I attributed this fact to an alteration 

 in shape of the bulb arising from the unequal pressures which 

 the current of air impressed upon it. The following experiment 

 proves that this is the true explanation. 



I emptied the mercury of my thermometer and replaced it by 

 alcohol. The instrument thus became far more sensitive to va- 

 riations in temperature, owing to the greater expansibility of 

 alcohol, but the variations produced by external pressure remain 

 the same. Now the thermometer which had thus been modified 

 never indicated a higher temperature than that of the bath, even 

 when I brought the reservoir into contact with the capillary orifice. 

 These experiments showed, moreover, that we should not use 

 mercurial thermometers to determine temperatures in rapid cur- 

 rents of gas; and I have had recourse to air- thermometers, on 

 the action of which the compression of the envelope exerts no 

 appreciable influence. 



Now the first fact I have thus observed is that, in all positions 

 of the thermometer placed in expanded air, this thermometer 

 indicates a lower temperature than that marked by the thermo- 

 meter placed in compressed air. The difference in temperature 

 is greater the nearer the bulb is to the capillary orifice ; the maxi- 

 mum is attained when the bulb is pressed against this orifice, 

 the surfaces not being sufficiently even to produce complete 

 closing. 



This latter fact is completely opposed to the elevations of 

 temperature noted by Messrs. Thomson and Joule. 



In a second series of experiments the bulb of the air-thermo- 

 meter placed in the expanded air had a larger diameter, so that 

 it entered the tube with slight friction, and remained there. 

 There was then a very narrow interval between this bulb and 

 the tube which surrounded it. I hoped thus to realize the con- 

 ditions in which the English physicists observed a great rise of 

 temperature. 



Now it is precisely under these circumstances that I obtained 

 the most cold ; it is then that the difference of temperature be- 

 tween the two thermometers varies least with the distance of the 

 bulb from the orifice. The explanation of these facts appears very 

 simple: in the present arrangement the air undergoes two suc- 

 cessive expansions — the first on emerging from the capillary ori- 

 fice, the second in the very narrow annular space of great dia- 

 meter which exists between the bulb of the thermometer and 



