37 



We are led, then, to the conclusion that both the colour 

 and crystalline form of this substance depend on the presence 

 of water in combination with Copper Sulphate, and this may 

 be clearly shown by the converse experiment. 



Placing on a plate some white Sulphate of Copper 

 (deprived of its water by means of heat), and pouring pure 

 water upon it, immediately the blue colour returns, the 

 added water recombining with the Copper Sulphate, and the 

 white amorphous powder regains at once its colour and 

 crystalline form. Water so combined with a crystal, that 

 when heat is applied to it it is driven off, leaving the crystal 

 amorphous, is termed Water of Crystallisation ; and such 

 water is generally driven off when the point of temperature 

 is reached at which water boils {i.e., loo' Centigrade, or 

 212° Fahrenheit). 



Another interesting fact, shewing the important part 

 water plays in determining crystalline form, is that one and 

 the same substance (for instance Carbonate of Soda), will 

 combine with different amounts of water, the crystalline 

 form being in many of these cases entirely different, according 

 to the amount of water of crystallisation present. 



The so-called Sympathetic Inks are now easily under- 

 stood. We take a weak solution of Cobalt Chloride in 

 water and brush it over cartridge paper. The characters 

 traced are practically invisible, for Cobalt Chloride, united 

 with its water of crystallisation, exhibits a pale pink colour, 

 but on warming the paper over a lamp the water of crystal- 

 lisation is driven off, and the Cobalt Chloride becomes dark 

 blue, the characters traced being now easily seen. If steam 

 were blown over the paper, the water of crystallisation 

 would be again taken up, the pink colour restored, and the 

 characters fade away. 



Barometric Flowers, very commonly seen some five or six 

 years ago, were cut out of linen and dipped in solution of 

 Chloride of Cobalt. When the air was laden with moisture 

 — rain impending — the flowers were pink ; if, on the con- 

 trary, the air was dry — portending fair weather — water 



