206 M. Achille Cazin on Internal Work in Gases, 



The maximum depression is about one-third of that in Series 

 IV., and the curves rise considerably above the axis of the ab- 

 scissae. We must remark that the curves of this latter series 

 affect the form a' V b" c' (fig. ]), although the water-manometer 

 communicated with the tubulure H of the rarefied reservoir. 

 This denotes friction in the tube which joined the two reservoirs. 

 Without this disturbing influence the depression would have 

 been greater in Series XII. In mentioning these experiments 

 (which were made at the outset of this research) I wished to con- 

 vey some notion of the difficulties which present themselves in 

 researches of this kind, and of the progressive course which has 

 led me to adopt the present apparatus, the results of which pre- 

 sent no uncertainty. 



§ XII. Influence of the nature of the gas. 



We are now going to compare with one another Series I. (air), 

 IX. (carbonic acid), and VIII. (hydrogen). All these series 

 were made with the apparatus ultimately adopted, and under 

 the same circumstances. Their curves are traced in figs. 4, 

 7, 6. The curves X X present considerable differences, which 

 we will examine. 



Let us consider the gases in the following order — hydrogen, 

 air, and carbonic acid. We see from figs. 6, 4, and 7 that the 

 maximum of h is nearly the same for the three gases ; but it is 

 reached more or less rapidly, so that the abscissae of this maxi- 

 mum go on increasing; the abscissas of the point where the line 

 X X cuts the axis of the x's also increase ; finally this line in- 

 clines more and more to the axis of the a^s. I conclude from 

 these observations that the point g (fig. 5) is lowest for hydrogen 

 and highest for carbonic acid. 



In fact, let us suppose the point g to be at the same height for 

 hydrogen and carbonic acid, and that the valve is opened at the 

 end of the time of (fig. 5) which corresponds to the point g. 

 Whilst the level of the manometer descends, the more rapidly 

 the pressure of the gas increases, the more quickly will the mi- 

 nimum b be reached, but the less will it have receded from the 

 axis of the a?9. The ordinate k b would thus be smaller for hy- 

 drogen than for carbonic acid, w T hich is contrary to observation. 



The point g cannot have the same abscissa for the three gases ; 

 this abscissa must increase from hydrogen to sulphuric acid, in 

 the same way as the abscissa o k of the point b. 



Thus when the stopcock which separates the two reservoirs is 

 opened, equality of pressure is established soonest with hydrogen, 

 latest with carbonic acid; and, moreover, the instant this equality 

 of pressure begins to take place, the pressure is smallest for hy- 

 drogen, and greatest for carbonic acid. 



