M. C. Lea — Endothermic Decompositions by Pressure. 417 



goes an intermediate conversion into heat. Rapid movements 

 are not needed, what is required is strong pressure with move- 

 ment, but this need not be rapid. Nor does the mortar or the 

 pestle become sensibly warm. The operation does not need to 

 be continuous but may be broken up with any number of in- 

 tervals. 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 subli- 

 mate to calomel. By heat, corrosive sublimate sublimes unal- 

 tered and the same is true of mercurous chloride. The three 

 silver haloids fuse unchanged at a red heat. The same conclu- 

 sion 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 gray 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 sub- 

 jected to a pressure of about 70,000 atmospheres absolutely 

 without change. It appears, however, that a pressure amount- 

 ing to less than a hundred pounds causes decomposition 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 triturated 

 in a mortar it first became yellow and then blackened without 

 difficulty. 



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



Mercuric Oxy chloride, 2HgO, HgC] 2 , obtained by precipi- 

 tating corrosive sublimate by potash acid carbonate, exhibited 

 the following reaction. Its brownish purple color 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 

 most be quite thin otherwise little effect is produced. 



Platinic Chloride. — Gradually darkened in a very marked 

 way, finally becoming blackish. 



Ammonium Platinichloride gave same result. 



