M. Achille Cazin on Internal Work in Gases, 197 



from those already arrived at by him. Such a needle moves 

 more quickly on the surface of water than below it. 



It must be remarked that the linear resistance does not de- 

 pend on the internal contour, but solely on those parts of it 

 which move normally or obliquely to their direction. The con- 

 tour of a circular disk which turns on its centre experiences little 

 or no resistance. The portions of the contour oblique to the 

 direction of their motion operate sensibly as their projections on 

 the normal to the direction of the motion are at equal distances 

 from the centre. 



Further experiments may lead to the introduction of some 

 modifications in the foregoing conclusions. 



XXIV. Memoir on Internal Work in Gases. 

 By M. Achille Cazin. 



[Continued from p. 99.] 



§ IX. Influence of moisture in the gas. 



SECTIONS V., VI., VII., VIII. indicate what are the indispen- 

 sable precautions for a good series of experiments. In the 

 following sections I shall adduce several facts which will prepare 

 for a complete explanation of the phenomenon. 



I shall first demonstrate that we cannot admit the depression 

 of the curve of the h's to be due to aqueous vapour in the gas. 



In order to ascertain the possible value of this objection, let us 

 examine the most unfavourable case — that in which the gas in 

 the reservoirs is saturated with aqueous vapour. 



The whole apparatus having a capacity of 42*868 litres, con- 

 tains 7*181 grms. of vapour at 20°, under the tension of 17 mil- 

 lims. of mercury, equivalent to 125 millims. of sulphuric acid. 

 When the whole of the gas in reservoir B is made to pass into 

 reservoir A, there is supersaturation in the latter, and a portion 

 of the vapour there is liquefied and deposited on the sides. If 

 the vapour which remains were condensed during the expansion, 

 forming a mist, the depression of h would have just the value of 

 125 millims., and the curve would afterwards rise in proportion as 

 the mist disappeared and the water condensed on the sides re- 

 turned to a state of vapour ; but this last effect would be slow, 

 and the depression would maintain itself a long time, probably 

 longer than in my experiments. 



Suppose the hygrometric state to be \ in the gas, and that 

 we compress it to 4 atmospheres in the reservoir A; there 

 would still be saturation in this reservoir, and the depression 

 would still take place ; but this time it could not exceed 31 mil- 

 lims. of mercury * and as there would be no water condensed on 



