ail apparatus for a liigli vacuum, consisting of a vessel witli 

 cocoa-nul carbon and a Geissi-kk tube, a tube for condensation of 

 tlie carbonic acid (by the aid of liquid air) and a \'essel of about 

 7, liter capacity as carl>onic acid reservoir ^). As it was exclusively 

 my purpose in these experiments to determine three-phase pressures, 

 a determination of the concentration of the mixtures used was 

 superfluous. For the lirst observation I used a mixture with great 

 excess of carbonic acid. When Ihe test lube had been screwed on 

 lo the pressure cylinder after the filling, and the mixture had been 

 heated to the ordinary (emperature, it appeared that the solid substance 

 could be kept at ordinary tein|)eralure only under iiigh pressure. 

 When the available volume was so small that there was only a small 

 quantity of gas presonl, then a three-phase equilibrium of compound 

 by the side of a thinly liquid layer (rich in carbonic acid) and gas 

 occurred at Ihe ordinary temperature at a pressure of about 

 50 atmospheres. When the pressure was diminished, the liquid 

 vanished with violent boiling, and solid remained l)y Ihe side of gas. 

 Below 30 almospheres Ihe solid sulistance decomposed with formation 

 of a liquid rich in aniline, a strong generation of gas t)eing perceptible 

 in this layer. Accordingly Ihe solid compound is deconq)Osed into 

 liquid and gas on decrease of pressure, ll is cleai' that the pressure at 

 which this decomposition just sets in indicates the three-phase coexistence 

 of compound, liipiid rich in aniline, and gas. On increasing enlargement 

 of the volume there remains coexistence of liquid by the side of gas. 

 At the ordinary temperature the existence of phenyl earbaminic acid 

 is, therefore, only possible at pressures above about 30 atmospheres. 

 Hence in perfect analogy with the gas hydrates the dissociation 

 tensions of this compound are Ihree phase equilibria. This applies 

 also to the determinations which Ditte carried out by observation 

 of the pressure at which gas begins to form from the crystals, or 

 of the pressure at which this generation ceases, which are of course 

 theoretically the same, but practically different according IoDitte'). 

 DiTTK ascribes the latter to inaccuracies of the temperature deter- 

 mination, in my opinion the slowness of the transformation S — >• L -|- ^ 

 is undoubtedly responsible for this. In my former researches on the 

 system sulphuretted hydrogen-water I have also been able to observe 

 such a slow transformation '). 



In order lo be able to determine Ihe three-phase pressures 



1) Cf. also These Proc. 13. 830 (1910/11). 



2) loc. cit. 

 ') loc. cit. 



