OF MICROSCOPIC OBJECTS. 129 



renders their durability uncertain, as sealing-wax, varnish, 

 liquid-glue, e. Dr. Bastian says the best c&mtmt far lignfrj 

 cells is one, much used in Germany, made by adding a 

 considerable quantity of nitrntp of hifitnm|i to a solution of 

 guni mastjcj rM nrnfn rm It can be procured at almost 

 any optician's. 



The student may feel himself at a loss in choosing the 

 cement which will give him the safest cells, many of them 

 becoming partially or wholly dry in a year or two, as stated 

 in another place. I can only give him a few general direc- 

 tions, and he must then use his own judgment. Of course 

 it would be lost labour to employ any cement upon which 

 the preservative liquid has any action whatever. It is also 

 a good rule to avoid those in whose composition there are 

 any particles which do not become a thorough and intimate 

 portion, as these unreduced fragments will almost certainly, 

 sooner or later, prepare a road by which the liquid will 

 escape ; and, lastly, whatever cement he uses, the cells are 

 always better when they have been kept a short time before 

 use. 



GUTTA-PERCHA RINGS have been recommended by some, 

 as affording every facility for the manufacture of cells for 

 liquids ; but they cannot be recommended, as, after a certain 

 length of time, they become so brittle as to afford no safe- 

 guard against ordinary accidents. Some have also used 

 india-rubber bands thickly coated with various varnishes ; 

 but these I consider less trustworthy than gutta-percha, 

 as they become thoroughly rotten in ordinary use after a 

 short probation. 



Often the cells must necessarily be of a large size, and 

 for this reason are made by taking four strips of glass of the 

 thickness and depth required, and grinding the places where 

 these are to meet with emery, so as to form a slightly 

 roughened but flat edge. The glass strip must also be 

 ground on the side where it meets the plate, and each piece 

 cemented with the marine-glue mentioned in Chapter II. in 

 the following manner: On that part of the glass to which 



K 



