472 Mr. M. Carey Lea on Transformations of 



therefore, which are taken. Ditte, in his work Les Mdtaux 

 (Fascicule ii. p. 500), adopts the numbers 21*1 and 15*5 re- 

 spectively. 



On this basis the amount of energy that must be supplied 

 to convert 2 HgO to Hg 2 + (endothermic) is that which 

 corresponds to 9*9 great calories. Therefore 400 grams of 

 mercury existing in the state of mercuric oxide will require 

 that amount of energy supplied to reduce it to mercurous 

 oxide. One gram, therefore, will require the equivalent of 

 24*75 small calories and one milligram "02475. 



In the experiment described mercurous oxide corresponding 

 to 30*5 mgs. metallic mercury was obtained. The energy 

 required to reduce 30*5 mgs. existing as mercuric oxide to 

 mercurous corresponds to 0*755 water-gram-degree and this 

 again to 321*5874 gram-metres. This number, therefore, 

 322 gram-metres, represents the mechanical energy trans- 

 formed into chemical energy in the above operation. 



Potassium Ferricyanide. — A good method of obtaining this 

 salt absolutely free from ferrocyanide is to dissolve the com- 

 mercial product in hot water and add a little potassium per- 

 manganate by degrees until the solution takes a faint reddish 

 colour. Yery 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 Ammonia Alum. — A specimen of this salt which, 

 when tested with potassium ferricyanide, gave a pale wine- 

 coloured solution without a trace of green, was taken and 

 3 decigrams 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 colour. 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'' 

 interval completely changes the reactions owing to the reoxi- 

 dation of tbe 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 colour 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 



