( 505 ) 
hydrogen bromide are capable of forming a compound. As in binary 
systems the safest conclusions as to the existence or non-existence of 
a compound may be drawn from the course of the melting point 
curves we have attempted to determine the melting point figure of 
the system HBr-Br. 
It soon became evident that, at atmospheric pressure, the hydrogen 
bromide instantly escaped from the mixtures so that we were com- 
pelled to use sealed tubes. The experiments were now carried out 
as follows: A quantity of specially purified bromine was weighed in 
a glass tube a part of which was drawn out; the tube was now 
connected to a HBr-generating apparatus and placed in a bath of 
solid carbon dioxide and alcohol. As soon as a sufficient quantity of 
HBr had condensed the tube was sealed and reweighed. The hydrogen 
bromide which was prepared from bromine, phosphorus and water 
was dried by passing it through two U-tubes containing P,O, whilst 
care was also taken that no moisture could enter the tube during 
the condensation. The tube was now fixed in a frame of copper 
wire and suspended in a rectangular wooden case, the long sides of 
which consisted of glass panes; in order to get a better isolation a 
second pane was fixed to each of these. Inside this case was placed 
a mixture of calcium chloride and ice for the higher temperatures 
whilst for the lower ones down to — 50° solid carbon dioxide and 
alcohol were used. For still lower temperatures this apparatus is 
unsuitable and the ordinary vacuum vessels were used; these, how- 
ever, suffer from the disadvantage that, unlike in the other apparatus, 
the tubes cannot be shaken properly without lifting them out. Any- 
how, in all cases we allowed the temperature of both to rise very 
slowly and the reading of the thermometer was taken at the moment 
that the last crystals fused. If only care be taken that the bath is 
kept constant at a trifling lower temperature for some time and that 
the tube and the bath are well stirred we may assume that the 
temperature of the mixture is practically the same as that of the 
bath. The observations were made with an “Anschütz” thermometer 
down to — 40° and a BaupiN toluene thermometer for the lower 
temperatures ; each determination was repeated a few times and 
the subjoined figures represent the mean result. 
Before stating our results we just wish to explain, that, strictly 
speaking, we do not determine a melting point curve by means of 
the method described, for a vapour phase is also existent in the 
tubes which deviates considerably in composition from the liquid, 
and exists perhaps under a relatively high pressure. And from the 
weighings we know only the total concentration, and not that of 
