OM CEMENTS. 275 



Asphaltum. — This, which forms a very good cement, is made 

 by dissolving Egyptian asphaltum in boiling linseed-oil or in 

 turpentine ; it answers very well for the first coating, but has 

 not suificient body, unless mixed with some solid material, to 

 form the entire mass of cement around the covers of objects ; 

 it IS of a fine black colour, and as such will serve for the last 

 or finishing coating. A solution of this substance in tur- 

 pentine will make a good cement for fastening cells to the 

 glass slides, instead of the marine-glue or Canada balsam, 

 having this advantage, that spirit may be employed as the 

 preservative fluid without injury to it. 



Canada Balsam. — A solution of this substance, either in 

 ether or turpentine, evaporated to such a consistence as is 

 suflScient to allow of its being laid on with a camel's-hair 

 pencil, has been recommended by Dr. J. W. Griffith as a very 

 good substitute for the gold-size; a mixture of lamp-black 

 and white hard varnish, when laid on immediately, he also 

 considers a good cement. 



Marine-Glue. — This most useful cement, the invention of 

 Mr. Jeffery, is composed of a mixture of shell-lac, caoutchouc, 

 and naptha ; many kinds are made, but that known in com- 

 merce as G K 4 is the best for microscopic purposes ; a solvent 

 for it is supplied at the manufactory,* and wUl be found of 

 great service to those who wish to construct any of the larger 

 kinds of ceUs or boxes. When about to be used, the glue 

 must be cut into thin slices, laid on the glass, and heated 

 until it begins to boil, or a hot iron may be placed on one of 

 the surfaces of the glass until the same effect is produced ; if 

 any gritty particles be present, they must be removed. To 

 ensure a firm connection between two surfafces of glass, both 

 must be well warmed, otherwise the glue will stick to one and 

 not to the other. The excess may be removed readily before 

 it is quite cold by means of the chisel described at page 272, 

 and all trace of the remainder by the employment of caustic 

 potash, care being taken that none of the latter be left in 

 contact with the glue, as it is apt to insinuate itself between 

 * Commercial Road, Limehouse. 

 18* 



