514 Mr. S. U. Pickering on the Heat of Combination 



The freezing-point of the tetrabromide is no doubt lowered 

 by the addition of water, and probably very rapidly so, for 

 with 1\ per cent, of added water no crystallization was 

 obtained at —70°; but with this amount of water, and also 

 with amounts up to 24 per cent., mutual solution is not com- 

 plete, the liquids being always cloudy. When the water is 

 increased to about 26 per cent., the liquid is clear at high 

 temperatures, and generally becomes cloudy on cooling (it 

 did not do so in the experiment at 73*4 p. c. SnBr 4 ); and the 

 more the added water is increased the lower is the tempera- 

 ture which the solution will stand without becoming cloudy. 

 Hard, well-defined, transparent crystals were obtained from 

 solutions containing 73*6 to 66*7 per cent, of the tetrabromide; 

 but with the strongest of these solutions the liquid was turbid 

 when solidification occurred, and the observations were diffi- 

 cult and doubtful. From solutions containing less than 56 

 per cent, of the bromide, water crystallizes. On plotting out 

 the results from 73*6 to 66*7 per cent, they form the interme- 

 diate curve shown in fig. 1, which on a more open scale is 

 found to indicate a maximum at a strength of 75*0 per cent* 

 of the bromide, and at a temperature of 19° ; that is, some- 

 what beyond the point at which the determinations become 

 impossible owing to the cloudiness and dissociation of the 

 solution. This indicated that the hydrate was probably an 

 octohydrate, which contains 75*261 per cent. To establish 

 this more satifactorily, a quantit}- of a 73-per-cent. solution 

 was allowed to deposit a few crystals, and these after being 

 drained were analysed and gave the values 



Sn, 20662, Br, 35-011, H 2 (by difference), 24*327, 



.theory for SnBr 4 8H 2 requiring 



Sn, 20-334, Br, 34*926, H 2 0, 24*739. 



This octohydrate, as may be inferred from what has been 

 said above, cannot be melted without becoming decomposed 

 and cloudy ; but if heated to a higher temperature till clear, 

 ?and then cooled without stirring, it may generally be cooled 

 to atmospheric temperatures and crystallized without decom- 

 posing. By repeatedly crystallizing in this way a sample 

 was prepared for the determinations. The crystals them- 

 selves, however, on being kept for any length of time, or on 

 being scratched, generally become cloudy and dissociated. 



The heat-capacity and heat of fusion determinations with 

 this substance could not be made in the ordinary manner, as 

 it would have been impossible to see whether, on cooling in 

 the platinum bottle, it had decomposed or not. Fortunately 

 it easily superfuses, and by taking advantage of this property 



