38 



passed from the flowers to the air, leaving them blue, and 

 from the colour the coming weather could be predicted with 

 some certainty. 



Another property of crystals deserves attention. We 

 take some crystals of Chloride of Calcium and place them 

 on a glass plate, placing the whole in one pan of a balance, 

 adding weights to the other pan until equilibrium is attained. 

 The crystals are now dry and the balance true, but almost 

 immediately they become damp, absorbing moisture greedily 

 from the air, and the balance is disturbed, more weights 

 being required to compensate for the moisture taken up from 

 the air. This absoFptive property is termed Deliquescence, 

 and is made use of to keep valuable apparatus dry and free 

 from rust ; for if a pan of Calcic Chloride is placed under 

 the glass case of the instrument, the air is kept constantly 

 dry, all moisture being taken up by the deliquescent body. 



Again, to obtain gases dry, it is only necessary to pass 

 them through a long glass tube loosely packed with frag- 

 ments of the same substance (Calcic Chloride). Some 

 crystals exhibit an exactly opposite tendency, thus Sulphate 

 of Soda (Glauber's Salt, discovered by him in 1658) is, when 

 first exposed to air, perfectly colourless and transparent, but 

 quickly becomes white and opaque, owing to water passing 

 Jrom the crystal to the air — such crystals are said to be 

 Efflorescent. 



Mr. Bloxam then demonstrated some of the methods by 

 which crystals may be quickly obtained. 



(i.) By Sublimation. 



In this method a volatile substance is converted by 

 means of heat into vapour, which is condensed by contact 

 with a cold surface yielding a crop of crystals. 



Place in a large glass flask some powdered Iodine, and 

 warm the bottom of the flask over a lamp. A magnificent 

 violet vapour rises (whence Iodine takes its name), and on 

 meeting the cool upper portion of the flask, deposits very 

 brilliant grey-black crystalline plates of pure Iodine. Again, 

 suspend a bunch of leaves and fern in the top of a glass bell- 



