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quantity of Hg Bry is present, the interval of temperature through 
which the transformation takes place is very considerable. The 
change, further, is subject to great retardation. For this reason the 
determination of the transition interval was not possible either by 
the dilatometric or by the thermometric method. 
By observing the change of colour it was possible to determine 
the composition at which the red crystals were completely converted 
into the yellow crystals at a given temperature. The circumstance 
that yellow mixed crystals containing up to 20 pCt. in molecules 
of Hg Br, may be converted into the red modification by grinding 
at the ordinary temperature was here used. 
The beginning of the change of the red into yellow crystals 
could not however be observed in this way. In order to do this the 
erystals which are deposited from mixed solutions at constant tempera- 
ture were studied. 
By ailowing mixed crystals to deposit from a sufficient number 
of solutions, a. solution is finally found from which both red and 
yellow mixed crystals are deposited and which therefore represents 
the two points of DE and DF which lie on the same temperature line. 
Theoretically, the nature of the solvent shouid have no influence 
on the result. This conclusion was confirmed by the results of 
experiments with alcohol and acetone as solvents. In spite of the 
fact. that. the solubilities of the two mercury salts and their ratio 
were very different, the same values were found for the coexisting 
yellow and red crystals. 
Even by this method, however, it was impossible to determine 
points on DE and DF below 0°. The direction of the two lines 
indicates that if HgBr, possesses a transition point, it is probably 
at a very low temperature. Experiments in this direction gave no 
indication of a transition down to —83°. 
It is to be remembered that even at ordinary temperatures, and 
much more so at higher temperatures, solid Hg Br, and Hel, diffuse 
into each other, so that the transition temperature of a finely ground 
mixture agrees closely with that which would be found for mixed 
crystals containing the same proportion of Hg Bry. 
Finally it was possible to show, by means of the known heat 
of transformation of mercuric iodide, that for moderate admixtures 
of He Brg the course of the lines DE and DF agreed with a formula 
which RorHMmurp has recently given for the case that the concen- 
trations of the two coexisting phases are known. This is the first 
case in which it has been possible to verify the laws of dilute 
solutions in the case of the relationship between two solid solutions, 
