M. Achille Cazin on Internal Work in Gases. 279 



of small jets, having certainly very little vis viva ; and when the 

 pressures were perfectly constant, this thermometer indicated a 

 constant temperature considerably less than the external tem- 

 perature. 



The general effects correspond to the theory ; but I do not 

 think that the method is susceptible of very great accuracy. The 

 influence of the sides and the porous partition on the thermometer 

 was very great (as the experimenters observed) when there were 

 variations of the pressure p { . Besides, the gas was not com- 

 pletely dry, and the correction of the effect due to moisture not 

 very accurate. Finally the position of the thermometer must 

 have had an enormous influence. Very close to the partition the 

 jets were still animated by a certain velocity, and the temperature 

 must have increased very rapidly from the partition to a certain 

 distance, from which it again became equal to the external tem- 

 perature. It appears evident to me that the effect observed on 

 the thermometer does not indicate the temperature which the 

 gas would have if it returned to rest after the expansion without 

 external calorific action. I think that the thermometer indicated 

 too low a temperature, and that hydrogen would have been able 

 to produce analogous effects to those of air under the same cir- 

 cumstances, as occurred in my own experiments. Unfortunately, 

 particulars of the observations made on this gas are not given. 



Neither is the method employed by M. Hirn* free from all 

 objection. Aqueous vapour on leaving the boiler passes through 

 a tube of 5 centims. diameter, where it is superheated; it 

 passes through an orifice of 4 millims. into a cubical wooden 

 box, which is enveloped by two other boxes. Thus the expanded 

 vapour circulates in the spaces between the boxes before issu- 

 ing into the atmosphere; and consequently the central cavity 

 is conveniently sheltered from the cooling action of external 

 bodies. In this cavity the thermometer is placed ; it is perfectly 

 sheltered by a partition against the direct shock of the jet of 

 vapour, so that the molecules of vapour only impinge upon it 

 after they have lost their velocities. But, on account of the mag- 

 nitude of the orifice, the molecules of vapour have a great velocity 

 on each side of the orifice, and there is a great fall in the tem- 

 perature, due to the velocity, between the orifice and the parti- 

 tion. Hence the radiation of the sides is considerable, and the 

 jet only returns to rest after having received heat from without ; 

 and therefore the final temperature observed on the thermometer 

 is too high. This objection was made by M. Combes f. 



* Exposition analytique et experimentale de la Theorie mccanique de la 

 chaleur, second edition, p. 177 (1865). 



t Expose des principes de la Theorie mccanique de la chaleur, par M. 

 Combs, p. 238(1807). 



