150 BRIDGMAN. 



transition line has a maximum temperature. HgBr^ under ordinary 

 conditions has a form isomorphous with the high temperature form of 

 Hglo, while the form of HgCl2 is not isomorphous with either the iodide 

 or the bromide. It would be interesting to tr\^ the effect of pressure 

 on the bromide and the chloride. 



Hg2l2 is a substance listed in the Tables of the French Physical 

 Society as dimorphous enantiotropic, with a transition point at 70°, 

 on the authority of Yvon, in 1873. I have been unable to find the 

 reference, but there is later work by Varet.^^ He recognizes three 

 forms in all, but probably two of these forms are unstable and there is 

 no enantiotropic transformation point. One variety is yellow-green; 

 it is precipitated under some conditions, but is very unstable. The 

 other new variety is red; it is unstable under ordinary conditions. 

 The reaction from the ordinary yellow to this begins at about 70°, 

 but is not complete until 245° is reached. It does not seem to be a 

 transition of the ordinary type. In the present work, no new form was 

 found up to 12000 kgm. at room temperature, or to 13000 at 125°. 

 In a further search for the second modification, Hg2l2 was heated in a 

 dilatometer at atmospheric pressure. Between 57° and 83° there can 

 be no discontinuity of volume of so much as one part in 3000. The 

 only evidence that I have found for the transition is a fairly rapid 

 deepening of the yellow color to a brown, on heating through 70°. 

 This evidence cannot be regarded as sufficient, however. 



AgCl and AgBr are cubic and crystallize isomorphously with each 

 other and the high temperature modification of Agl. We would 

 expect therefore to find at low temperatures at atmospheric pressure 

 a transition of each of these substances to another form isomorphous 

 with the low temperature modification of Agl. No such transition 

 point seems to be known, and I do not know whether the search has 

 been made. Analogously one would expect at higher pressures at 

 room temperature to find another modification of AgCl and AgBr 

 corresponding to Agl (III). No such form was found, however, nor 

 was there anything at 200°. In my previous paper I referred to the 

 low temperature modification of Agl as hexagonal. There are many 

 authorities for this, but the most recent work ^^ seems to establish 

 that this is cubic, although imitating very closely the hexagonal form 

 by its peculiar method of twinning. 



The series of three salts NH4CI, NH4Br, and NH4I, is the most 



31 R. Varet, Ann. Chim. Phys., 8, 79 (1896). 



32 F. Wallerant, Cristallographie, p. 275. 



