86 M. Achille Cazin on Internal Work in Gases. 



son obtained results which accord with the thermodynamic for- 

 mulae : for carbonic acid the fall was 8°*33 for an excess of 

 pressure of 6'95 atmospheres at the ordinary temperature. For 

 aqueous vapour at a temperature of 244° and with an excess of 

 pressure of 6 atmospheres, M. Hirn obtained 233° after the ex- 

 pansion, and consequently a fall in temperature of 11°. He also 

 found a satisfactory agreement between his results and the for- 

 mulae to which he had been led. 



This method is not free from every objection ; it will be com- 

 pletely discussed in the course of this memoir. 



These two methods are certainly useful in revealing the exist- 

 ence of attractive molecular forces in gases ; but in practice a 

 host of circumstances may complicate the phenomena, and it 

 appears to me that it will be useful to submit them to new 

 tests. The experiments which I am about to describe will show 

 how the expansion takes place in the case of two reservoirs. 

 They form a sequel to the experiments which I have described 

 in the Annates de Chimie et de Physique*. I observed at that time 

 the passage of gas from one reservoir to the other when the ex- 

 cess of pressure was small, and I devised a method for obser- 

 ving the condition of the gas at each moment during the 

 passage. By using at the present time an analogous method, 

 I have succeeded in observing all the particulars of the flow of gas 

 under a great pressure. Hence this method differs essentially 

 from those methods usually employed. Instead of simply in- 

 vestigating the pressure which is established after the motion of 

 the gas has completely ceased, I investigated it at several stated 

 periods during the motion ; and by the comparison of various 

 observations I deduced the approximate time of the cessation of 

 the motion, and the pressure of the gas at that time. This is 

 evidently a suitable method whenever a temporary phenomenon 

 has to be observed in which the quantities we wish to measure 

 vary very rapidly with the time. By this contrivance pheno- 

 mena as fugitive as the movements of a gaseous mass are ren- 

 dered accessible to observation. 



I shall devote the first part of this memoir to a description of 

 the experiments, and to the conclusion which may be immedi- 

 ately deduced from them without having recourse to any mathe- 

 matical theory. In the second part I shall apply the thermody- 

 namic formulas to the explanation of the facts observed. 



* 'Essai sur la detente et la compression des gaz sans variation de 

 Chaleur," Ann. de Chirn. et de Phys. S. 3. vol. lxvi. p. 206 (1862). 



