OF MICROSCOPIC OBJECTS. 129 
renders their durability uncertain, as sealing-wax, varnish, 
liquid-glue, &c. Dr. Bastian says the best cement for liquid 
cells is one, much used in Germany, made by adding a 
considerable quantity of nitrate of bismuth to a solution of 
gum mastic in chloroform. 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 Rives 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 trust-worthy 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 
