450 MANIPULATION. 



are required to be kept, they should be mounted either in the 

 dry way or in Canada balsam ; in order to preserve them from 

 being injiured by the pressure of the cover, a cell of paper, 

 cardboard, or glass, should be placed round them, and the 

 cover cemented, as before described, at page 313. According 

 to Mr. Fox Talbot, a solution of sulphate of copper, to which 

 a small portion of nitric ether has been added, will crystallize 

 in the form of rhomboids ; these, when viewed under polarized 

 light, resemble brilliant rubies, emeralds, and other gems; 

 according to the same gentleman, the oxalate of chromium 

 and potash, dissolved in water, and rapidly crystallized, is a 

 splendid object. Sir David Brewster recommends the Faro 

 ApophyUite, when the prisms are complete, as exhibiting 

 most gorgeous colours. K it be wished to examine the 

 crystals of any salts during their formation, the crystallization 

 should be carried on in a glass that is slightly concave ; one 

 of the cells represented by figs. 178-9 will be found to answer 

 the purpose, but the best apparatus of aU will be the small 

 concave discs of glass about a quarter-of-an-inch in diameter, 

 three or more of which, set in a frame of metal, were generally 

 supplied with all the old microscopes, and were employed for 

 containing infusoria, as well as for viewing the crystalhzation 

 of salts. All those crystals that are so thin as not to appear 

 light, or to exhibit colours when the field of view is made 

 dark by the position of the prisms, may have colour given 

 them when placed upon the selenite stage or when a film of 

 the same material is laid under them. Many organic sub- 

 stances also, though they do to a certain extent appear 

 luminous upon a dark ground, may be made to exhibit colours 

 when placed over selenite. K colour only be required to be 

 shown by any given object, a double image prism placed 

 over the eyepiece will be found the best to he adopted, 

 as then any preparation, whatever its structure, will exhibit 

 the effects of polarized light. The plan of preparing the 

 sections of whalebone, horns, hoofs, hairs, &c., has already 

 been given at page 341 ; and at page 333, the best method of 

 procuring the siliceous skeletons of grasses ; wldlst at page 

 400, the names of the finest specimens to select for the pur- 



