90 BRIDGMAN. 



intermediate one asymmetric, and the low temperature modification 

 rhombic. 



Carbon Tetrabromide. — This substance is not carried in stock by 

 any of the large chemical houses, but was made to order by Hoffmann 

 and Kropff, and guaranteed by them to be chemically pure. Two 

 separate lots were made by them, of about 100 gm. each. The second 

 lot was perhaps purified more carefully than the first, being " triply 

 distilled with steam," and showed a somewhat higher transition point. 

 The particular interest of this substance lies in its suspected poly- 

 morphic isomorphism with CCI4. In the first paper of this series two 

 new modifications of CCI4 at high pressures were found, and since the 

 writing of that paper, I have learned that a transition point at at- 

 mospheric pressure, on the prolongation of the transition line at high 

 pressures, has been discovered by Goldschmidt -^^ at —47°. Now it is 

 known that CBr4 is dimorphic at atmospheric pressure, the transition 

 being at about 47°. The suspicion was strongly suggested that the 

 transition point of CBr4 at 47° corresponds to the newly found one of 

 CCI4 at —47°, and that at high pressures another modification of CBr4 

 would be found corresponding to the third modification of CCI4. 

 As a matter of fact, another modification was found, but it does not 

 have at all the relation to the other two forms that is suggested by 

 CCI4. It is unfortunate that here, as in the case of CiClg, the sub- 

 stance begins to decompose just where things are getting most in- 

 teresting. 



Measurements were made with five difl^erent fillings of the apparatus. 

 With the first lot, two sets of readings at high pressures were made; 

 one at 200°, which was terminated by decomposition, and the other 

 to only 100°. A third filling with this lot was made for the low pres- 

 sure point. With the second lot, one filling was made for high pres- 

 sure measurements to 123°, and one filling for the low pressure point. 

 The quantity used varied from 35 to 59 gm. In all cases the sub- 

 stance was hammered cold into the inverted steel shell, and pressure 

 transmitted to it by mercury. 



These several runs- are perhaps worth describing in some detail, 

 both because the new modification belongs to a new type, and because 

 of the decomposition effects, which are interesting in themselves, and 

 which might without detailed description, be thought to make ques- 

 tionable the validity of the new transition. It was of course expected, 



10 V. M. Goldschmidt, ZS. Kryst. 51. 26 (1912). 



