THEORY OF SOLUTION AND CRYSTALLISATION. 255 



Nickel Chloride. 

 Yellow ... ... ... ... Ni CI2 



Green ... ... ... ... Ni Clj 9OH2 



Ferrous Iodide. 



Black ... ... ... ... Fe I2 



Green ... ... .. ... Fe I2 4OH2 



Etc. Etc. 



9. The existence of coloured compounds, such as those just 

 described, helps to explain the singular changes which solutions of 

 the salts referred to undergo when exposed to varying temperatures. 

 If we take a moderately diluted blue solution of copper chloride 

 and heat to boiling, it becomes green. Red solution of cobalt 

 chloride when heated becomes deep blue., and so on in other cases. 



It is impossible to avoid the inference, that the effect of the 

 higher temperature is to cause the salt to part with a portion of its 

 water. 



10. An intimate connection may be at once perceived between 

 such phenomena and the facts observed in the crystallization of 

 salts and other compounds which are capable of taking up water 

 of crystallization. The amount of such water combined with the 

 salt is well known to be dependent solely on the temperature. 

 There is no more remarkable instance of this than the sulphate of 

 manganese, which, on the authority of Kopp, is represented as 

 capable of crystallizing with four different proportions of water. 

 The composition of the crystals is given in the following 

 formulae : — 



Temp, of Crystallization, Composition of Crystals. 



Below 6° ... Mn SO4, 7 OH2. 



6^ to ic*=' ... Mn SO4, 5 OH2. 



2o« to 40'' ... Mn SO4, 4 OH2. 



Near 100° ... Mn SO,, 3 OH2. 



Many salts habitually crystallize in the anhydrous state, but 

 that this is only an accident arising from the relatively high 



