Decompositions obtained by Pressure. 35 



others which remain to be described, mechanical energy- 

 undergoes an intermediate conversion into heat. Rapid 

 movements are not needed ; what is required is strong pres- 

 sure with movement, but this need not be rapid. Nor does 

 the mortar or the pestle become sensibly warm. The opera- 

 tion does not need to be continuous, but may be broken up 

 with any number of intervals. But a decisive conclusion can 

 be drawn from those cases in which decompositions are 

 effected in this way that cannot be produced, by heat. For 

 example, in the next instance to be mentioned there is a 

 partial reduction of corrosive sublimate to calomel. By heat, 

 corrosive sublimate sublimes unaltered, and the same is true of 

 mercurous chloride. The three silver haloids fuse unchanged 

 at a red heat. The same conclusion can be drawn from 

 other reactions. 



These results were obtained in an atmosphere absolutely 

 free from dust, so that the reducing action of this substance 

 was completely excluded. 



Mercuric Chloride. — A specimen which, after lightly pow- 

 dering, did not darken in the least with ammonia, was 

 triturated in the manner just described with several intervals, 

 in all for 15 minutes. It then became grey in a very striking 

 way when moistened with ammonia. 



This is a very interesting reaction. In the first part of 

 this paper it was mentioned that mercuric chloride could be 

 subjected to a pressure of about 70,000 atmospheres absolutely 

 without change. It appears, however, that a pressure 

 amounting to less than a hundred pounds causes decomposi- 

 tion when combined with movement, showing the enormously 

 greater efficiency of shearing-stress as compared with simple 

 pressure. Not only this but, as just mentioned, shearing- 

 stress produces decompositions which heat is not competent 

 to effect. 



Mercurous Chloride. — When calomel was sharply tritu- 

 rated in a mortar, it first became yellow and then blackened 

 without difficulty. 



Turpeth Mineral, 3HgO,S0 3 . — Reduces rather slowly. 



Mercuric Oxy chloride, 2HgO, HgCl 2 , obtained by precipi- 

 tating corrosive sublimate by potash acid caroonate, exhibited 

 the following reaction. Its brownish-purple colour by light 

 grinding became lighter, and then when strong force was 

 used it blackened with remarkable ease. 



Mercuric Iodide shows a trace of blackening. 



Mercuric Oxide. — This substance yields much more readily 

 to trituration than to simple pressure. Especially on the sides 

 of the mortar it was quite blackened. The layer of material 

 must be quite thin, otherwise little effect is produced, 



P2 



