450 



4. Vapour pressure measurements. 



It follows from the observations of the vapour pressure of solid 

 ammonium chloride through extrapolation that the detection of the 

 transition temperature through observation of a discontinuity in the 

 vapour pressure line would require an exceedingly accurate pressure 

 measurement ; the pressure at 180° only amounts to a few milli- 

 meters of mercury. 



I have, therefore, tried to find a discontinuity in the three-phase 

 line SLG of the system NH^CI— HjO. For if we measure the vapour 

 pressures of the saturate solutions, the transition temperature will 

 remain unchanged, at least if the solid substance does not absorb 

 water in appreciable quantities. The vapour pressure measurements, 

 performed by means of Cailletet tube and air manometer according 

 to the well-known method, yielded no break which could be demon- 

 strated with certainty when the accuracy was about Vso ^'m- As 

 I however want these vapour pressure measurements for the deter- 

 minations of § 5, I have inserted some of the found pressures in table 1. 



5. Determinations of the solubility of ammonium chloride in loater 

 at temperatures between 160° and 205°. 



Another method for the determination of the transition point 

 is found in the determination of the discontinuity in the liquid 

 branch of the above mentioned three phase line SLG in the system 

 NH,Cl— HjO. It is known that the liquid points can only be deter- 

 mined by approximation directly by means of fused tubes. In a 

 liquid point the system can namely exist entirely as liquid phase 

 which is just saturate with solid substance, the pressure being exactly 

 equal to that of the vapour which might coexist with the liquid 

 phase. If we observe in a tube the vanishing point of the crystals, 

 we determine the liquid point of a mixture indicated by the con- 

 centration of the liquid at the disappearance of the last crystals; we 



