OF MICROSCOPIC OBJECTS. 85 



which will give him the safest cells, many of them becoming par- 

 tially or wholly dry in a year or two, as stated in another place. 

 I can only give him a few general directions, 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 pafticles 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, as already stated. 



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 safeguard against 

 ordinary accidents. 



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 men- 

 tioned in Chapter I. in the following manner : On that part of 

 the glass to which another piece is to be attached should be laid 

 thin strips of the glue ; both pieces must then be heated upon a 

 small brass table, with the aid of the spirit-lamp, until the strips 

 become melted ; the small piece is then to be taken up and placed 

 upon the spot to which it is to be attached, and so on until the 

 cell is completed. It will be found necessary to spread the glue 

 over the surface required with a needle or some other instrument, 

 so that an unbroken line may be presented to the wall of the cell, 

 and no bubbles formed. Too great a heat will " burn " the ma- 

 rine-glue, and render it brittle ; care must be therefore taken to 

 avoid this. 



When shallow cells are required, those which are made by 

 grinding a concavity in the middle of an ordinary slide will be 

 found very convenient. The concavities are cut both circular and 



