POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 93 



ners, this sample is evidently purer, although it was not perfectly pure, 

 because there was perceptible rounding for this also. The agreement 

 of the two samples at high pressures is better than at atmospheric 

 pressure; doubtless considerable of the impurity shown by the first 

 sample at low pressures was due to a gradual decomposition, and did 

 'not affect the values at high pressures, which were determined nearer 

 to the time of preparation of the sample. 



The decomposition phenomena are of special interest; apparently 

 there are two kinds of decomposition. The first sample showed both 

 kinds. The first kind of decomposition is one with increasing volume 

 and took place in notable degree for the first sample at 176°. This is 

 evidently an effect of the high temperature, and not of the pressure. 

 The second decomposition is one with decreasing volume, and was 

 shown by the first sample at 200° and about 7000 kgm. Because of 

 the decreasing volume, we can tell that this is probably a decomposition 

 brought about by the high pressure and not by the high temperature. 

 Such decompositions are not usual; this is the first case of it I have 

 found. Apparently the result of this decomposition with decreasing 

 volume is complete resolution into carbon and chlorine. The second 

 sample showed only the first kind of decomposition with increasing 

 volume at 145°; at this temperature the decomposition ran very nearly 

 to completion, only a small trace of the II-III transition being found 

 on decreasing pressure. The apparatus was cooled and taken apart 

 after the decomposition at 146°. There was some free bromine, but 

 apparently no free carbon. The larger part of the mass was a 

 white solid, but several cu. cm. of a colorless liquid had been formed. 

 After this run, the steel shell with the CBr4 was sealed into a glass 

 tube to preserA'e the decomposition products. The colorless liquid 

 was unstable; after several weeks it had entirely disappeared, its 

 place being taken by hexagonal crystalline plates. This is not the 

 crystalline form of either of the forms of CBr4. The chemistry 

 of this decomposition might prove an interesting subject for investi- 

 gation. 



As regards the accuracy of the results given above, the jj-t values 

 are probably pretty good, but the AT' values are too high on the I-III 

 and the II-III curves. This is the result of the slow decomposition, 

 even at low temperatures. It is known that at atmospheric pressure 

 C'Br4 decomposes at 100°, so that the decomposition found here is not 

 surprising. As a result of the high AF values, the AH values are also at 

 fault. There can be no question of this, because they do not check at 

 the triple point. There seemed to be no way of adjusting the ^■arious 



