Mechanical into Chemical Energy. 379 



gram meters, represents the amount of mechanical energy 

 transformed into chemical energy in the above operation. 



Potassium Ferricyanide. — A good method of obtaining 

 this salt absolutely free from ferrocyanide is to dissolve the 

 commercial product in hot water and add a little potassium 

 permanganate by degrees until the solution takes a faint red- 

 dish color. Very little is usually required. The solution is 

 then filtered and crystallized, taking the first crystals only. 



Ferricyanide purified in this way, was triturated about 20 

 minutes in an agate mortar. On adding water a deep green 

 solution was obtained which by standing and warming let fall 

 abundance of a blue powder. 



Ferric Ammo7iia Alum.. — A specimen of this salt which, 

 when tested with potassium ferricyanide gave a pale wine col- 

 ored solution without a trace of green, was taken and 3 deci- 

 grams were triturated for 25 minutes in a porcelain mortar. 



After trituration the ferrous salt formed distinctly reduced 

 gold solution. A few drops of the dissolved substance being 

 added to a solution of ferricyanide changed it to an intense 

 green color. Undoubtedly reduction had taken place. It 

 need scarcely be mentioned that in this case the tests should be 

 applied immediately after the trituration. A few hours inter- 

 val completely changes the reactions owing to the re-oxidation 

 of the ferrous salt formed. 



This experiment was repeated with additional precautions. 

 To a strong solution of ferric ammonia alum, enough potash 

 permanganate was added to distinctly color the liquid which 

 was then made to crystallize quickly by cold. These crystals, 

 certainly free from ferrous salt, were then actively triturated 

 in a porcelain mortar for 25 minutes. The filtered solution 

 produced immediate purple clouds in a cold very dilute solu- 

 tion of gold chloride and an abundant blue precipitate in one 

 of potassium ferricyanide. 



It is essential that the ferric alum be thoroughly dried (at a 

 temperature not exceeding 40° C), otherwise owing to the large 

 quantity of water of crystallization it will become pasty in 

 grinding and then no reduction will take place. 



Cupric Chloride. — Even by long trituration this salt showed 

 no indication of reduction. 



This reaction taken with the preceding shows how distinct 

 is the action of mechanical energy from that of heat. For 

 cupric chloride is reduced by heat to cuprous chloride, but 

 shearing stress has no such action. On the other hand shear- 

 ing stress reduces ferric sulphate which heat does not. 



Sodium, Chloraurate. — In the previous paper, the effect of 

 triturating this salt in a porcelain mortar was given. For com- 

 parison it has since been tried in an agate mortar. With 



