200 M. Achille Cazin on Internal Work in Gases. 



The same result is obtained by comparing Series IV. and I. 

 (air). The coordinates of the minimum are A=— 35 millims. 

 (sulphuric acid) in Series IV. (glass), and A=— 136 millims. 

 (sulphuric acid) in Series I. (zinc). Moreover the abscissa of 

 the point ^ = is greater, and the curve does not rise so far 

 above the abscissse in Series IV. as in the other. 



The experiment shows thus that the cause of the depression 

 is intimately connected with the quantity of gas which the jet 

 encounters during the expansion. We must attribute this depres- 

 sion to a mechanical or thermic effect counteracted by the gas 

 contained in reservoir B. Let us examine how this can take 

 place. 



By distinguishing three parts in the gas in motion as we have 

 done before, we see, first, that the quantity of gas left in reservoir 

 A is the same in each pair of experiments, since the expansion 

 commences at the same pressure p x and finishes at the same 

 pressure p'. The quantity of gas which passes into reservoir B 

 is also the same, since the reservoir A always contains the gas 

 compressed under the pressure p r The third part, that which is 

 contained in reservoir B, alone changes; it increases with its 

 capacity. The law of expansion of the first part remains the 

 same ; the gas expands by overcoming a pressure equal at each 

 moment to its elastic force. But it is not the same for the other 

 two parts. The second always expands, it is true, from the pres- 

 sure p x to the pressure p 1 , but by overcoming a pressure less than 

 its elastic force and varying from p 2 to p' ; and we have seen 

 that p q increases with the capacity of B. The resistance to the 

 flow being smaller than the elastic force of the portion of gas 

 considered, its molecules acquire certain velocities; and evidently 

 these velocities are greater the less the counterpressure p 2 . This 

 is a difference of mechanical effect about which we can have no 

 doubt. 



The state of motion of the second part can only last a certain 

 time ; little by little the molecules lose their velocities, producing 

 heat, so that the mechanical effect finally transforms itself into 

 a thermal effect. Such a condition must contribute to the de- 

 pression observed in experiments where the value of is very 

 small. We are thus brought to recognize one of the causes of this 

 depression. The greater the velocity of the efflux the greater 

 the depression ; hence it is due, at least in part, to the fact that 

 the gaseous molecules do not instantaneously lose their velocities. 



The character of the third part remains to be studied. It is 

 compressed from p 2 to />', and consequently its temperature is 

 raised. The compression and rise in temperature, on the one 

 hand, are less when p 2 increases ;* but, on the other, the quantity 

 of gas which constitutes the third part increases more rapidly 



