( 539 ) 



vapour generally contains but little water, and we are dealing with 

 branch 2. 



The conversion now taking place with heat supply at a constant 

 pressure is: 



solid — *• liquid -|- vapour. 



In all those cases it is, therefore, not the liquid which boils but 

 the compound. The gas is very plainlj^ seen to emanate from the 

 crystals lying in the liqidd, whilst the latter does not diminish but 

 increases. The phenomenon has been very plainly observed with the 

 two hydrates of HCl and of H Br and with those of SO^ and Cl^. 

 With the last two and with HC1.H,0 it could be observed at 1 atm. 

 pressure. 



It must also exist with I CI but limited between 27° at 39 mm., 

 and 22° at 24 mm., much more plainly with I CI, where it may 

 appear between the melting point 101° at 16 atm. and 34° at 100 mm. 

 Between this a three-phase pressure of 760 mm. occurs at 64°, and 

 at the said temperature it may, tiierefore, be observed in an open 

 apparatus. Solid ICl, breaks up into a liquid with 63 and into a 

 vapour with 89 atom-percent of chlorine. 



That similar phenomena may also appear in compounds which are 

 very stable at a lower temperature, has recently been demonstrated 

 by Aten in the case of Bi^S,. This sulphide breaks up at 760° into 

 a liquid containing 55 atom-percent of S and a vapour consisting 

 almost exclusively of S. Therefore, the actual melting point of the 

 sulphide cannot be determined at 1 atm. pressure. A similar behaviour 

 may be expected of many compounds having a melting point situated 

 much higher than the boiling point of one of its components, such 

 as in the case of oxides, sulphides, phosphides etc. 



We must point out another peculiarity which distinguishes the 

 boiling phenomena on branch 2 from those on branches 1 and 3. 

 The liquids and vapours belonging to the latter are both either richer 

 in A or richer in B than the compound : consequently the boiling 

 phenomena concerned are observed in systems consisting of the com- 

 pound with a smaller or larger excess of one of the components. 

 On branch 2 however the vapour is richer in A and the liquid 

 richer in B, therefore the boiling phenomenon can occur in mixtures 

 of the compound with ^1 as well as with B. In the tirst case such 

 a system, below the 'boiling point at the existing pressure, consists 

 of compound -|- vapour and the liquid appears only at the boiling 

 point, in the second case, the system below the boiling point con- 

 sists of compound -f- liquid and the vapour appears at the boiling 



