272 MANIPULATION. 



scraped away easily with a small chisel, such as represented 

 by fig. 174, or by a knife, but if this operation be left until the 



Fig. 174. 



cell is cold, the task then will be more difficult. As soon as 

 the cell is cold, a small quantity either of a weak solution of 

 potaah, or even spirits of wine, may be poured into it, and all 

 the small particles of glue removed first with the chisel, and, 

 secondly, with a chisel-like piece of wood, having a thin 

 piece of rag over it, so that the cell may be freed both from 

 the cement and from any greasy matter that may be present 

 (care being always taken that the glass be not scratched). 

 The cell may now be rinsed out with clean water and wiped 

 dry, first with a cloth or an old cambric handkerchief, and 

 finished with chamois leather ; it is now ready for use. 



When the glue has been heated too much, it turns black, 

 becomes tough, and wiU not stick to the glass ; when this has 

 happened, it is better always to take away the cell, scrape oflF 

 all the old glue, and begin afresh, than attempt to cement 

 it with glue that has been over-heated and lost its fluidity. 

 As soon as the glue boils, no time should be lost in laying 

 down the cell in its proper place. Sometimes small black 

 gritty masses are present in the glue, these should be re- 

 moved whilst the melting is going on, as when they are left, 

 unless they can be crushed by pressure, they will prevent 

 the cell from coming down flat on the glass, and often large 

 air bubbles will appear when the pressure is taken oiF. There 

 are many kinds of marine-glue in use, but the best for 

 cementing cells is that known in commerce as G. K. 4, this 

 melts at a temperature some few degrees higher than that 

 of boiling water, the harder kinds get brittle by keeping, and 

 their melting point is much higher. While the plate is 

 warm, and all the materials are at hand, it is best to cement 

 a number of cells, all of which may be cleaned and put by 

 ready for use, as it will be found in practice necessary to 



