( 237 ) 



§ 3. DeteDn'uiatioii of the. nwlccuLir volunu' (>/ tluu'oexistliui phases. 

 As \vc (lid not aim at. a very liigii degree of iU'cnracy a few remarks 

 will be suffic'iem. As to the li({uid phase: with the limitations and 

 conditions to he treated in § 5, ])art of liie gaseous mixtui'e in the 

 a[)paratns ^, called for shortness the piezometer, may by conden- 

 sation attain a li(pu(l ])hase of a composition which, as said in § 'i, 

 is derived from a; of the oi'iginal gaseous mixture by a correction. 

 Measurements with the volnmenometer yield the volume that the 

 liquid phase would occupy in its gaseous state. And by means of the 

 divisions at />5 and />,. PI. I fig. 3 we read the volume of the liquid 

 phase itself. The molecular volume of the gaseous phase may best 

 be derived from the coexistence pressure and from isothermals which 

 with not too small pressures can be determined after the method 

 of Comni. N". 78, April '02, if necessary with the dew-point appa- 

 ratus ï^ itself. AVith not too large pressure it w\\\ in most cases be 

 possible to apply the correction for the deviation from the law of 

 Boyle-Gay-Lussac-Avoüadro by means of the empirical reduced equa- 

 tion of state (Comm. N". 71, June 'OJ) according to the law of cor- 

 responding states. 



§ 4. Bcgimiim/ of the condensation. In order to determine this we 

 have applied the principle of IIegnault's hygrometer '). To this Gn<\ 

 we observe the tirst condensation which is formed in a part of the 

 apparatus of which the temperature is a little below that of the 

 surrounding gaseous mixture when the gaseous mixture is brought 

 to the begin condensation pressure. In order to be able to observe 

 a very small condensation we have taken for the place of the lowest 

 temperature a shining mirror, and next to it is placed another mirror 

 which is not cooled. 



The apparatus is blown of one piece in the same way as an ice 

 calorimeter of Blnsen. The outer space a has a capacity of about 

 20 c.c. and is provided with a capillary to which the steel capillary 

 <j\, through which the gaseous mixture is led, may be sealed on. 

 One of the mirrors d is sealed on to the bottom. The inner tube Cj 

 carries at c^ the second reflecting surface and serves at the same time 

 as a cryostat to keep the temperature of this surface constant, a 

 little below the teniperature of the gaseous mixture. 



To this end we have devised in the same way as for the outer 

 cryostat the cover ///,, lifting hermetically on the tube c., the small 

 stirrer h^ of which the rod A.^ projects through an india-rubber tube 



1) Battelli, (Ann. dc Ghim. et de Phys, scr. G, vol. 25, p. 59, 1892), has found 

 that for piiro snbstnnces thn deposition of lifpiid on a not cooled mirror placed in 

 tlie vapour is a suitable means for Ihe determination of the condensation point. 



16* 



