PROCKEDINGS OF SOCIETIES. 119 
addition of glycerine to the solution: the proportions I have 
used are equal parts of gum, distilled water, and glycerine ; 
to prevent the growth of minute alge or fungi in the 
mixture I have added a little arsenic. The following is 
the formula I would recommend: Boil together, in a Flo- 
rence flask or porcelain capsule, 3 grains of arsenious acid 
and 2 fluid ounces of distilled water ; when cold filter through 
paper. ‘Take of this arsenical solution 1 fluid ounce, of pure 
glycerine 1 fluid ounce, of pure gum acacia 1 ounce (Troy). 
The gum should be dissolved without heat; a fortnight or 
longer will be required for its complete solution: in the 
mean time the mixture should be occasionally stirred with 
a glass rod ; it will be well not»to shake the bottle so as to 
froth the mixture, for air introduced is retained with great 
tenacity, and many days elapse before it entirely disappears. 
If due care be taken in selecting pieces of gum transparent, 
bright, and free from impurities, the mixture will not need 
filtering ; if, however, foreign matters have accidentally 
gained admission, the best substance through which to strain 
the mixture is fine cambric, through which a considerable 
quantity of clean, cold water has been made to flow, so as to 
wash away any dust or loose fibres of the fabric which might 
find their way mto the mixture. This is an almost saturated 
solution of gum; it has nearly the consistence and appear- 
ance of fresh, pale Canada balsam, and is to be used in the 
same way, but without heat. The portion of the liquid 
which extends beyond the thin glass cover, soon dries (the 
water rapidly evaporating), the residue being a tough elastic 
compound of gum and glycerine, strongly adhering to the 
glass, and with no tendency to crack. The superfluity may 
be cut away with a knife, and any remaining smear be.re- 
moved by a piece of soft rag moistened with clean cold 
water. The specimen may be left in this state like an ordi- 
nary ‘ balsam-mounted’ object; or the edges of the thin 
glass cover may be coated with any of the cements commonly 
used for that purpose, or (which I prefer) a piece of tinfoil, 
with a hole of appropriate size, may be placed over the cover 
and be cemented to the slide with a solution of Canada balsam 
in ether. The most delicate structures are well shown and 
preserved in this medium—such as thin sections of recent 
vegetables, starch corpuscles, mycelium, and sporules of fungi; 
cells, vegetable or animal; the thm, delicate membrane of 
small hydatid cysts, &e. Pathological specimens, so difficult to 
keep unchanged for more than a short time, have been better 
preserved in this medium than in any other with which I am 
acquainted ; cancer cells, for example, have been kept unal- 
