COLLECTION OF SPECIMENS 179 



dispensed with. The formula which I have 

 adopted is taken from Carpenter's work on the 

 microscope, and runs as follows: Take half an 

 ounce (by weight) of pure gum arahic, and half 

 an ounce (also by weight) of water ; dissolve the 

 gum in the water by occasional stirring, but 

 without the application of heat ; add a quarter 

 of an ounce of glycerine, and mix well ; add half 

 an ounce of arsenious acid (about } gr. to J oz.) ; 

 filter through fine cambric. 



Formalin. A three-per-cent. solution of for- 

 malin makes an extremely serviceable preservative, 

 and I have found this very useful where some of 

 the more delicate and " filmy " leaves form the 

 subject of the mount. It is frequently difficult, 

 in such a case, to use glycerine jelly, as the 

 leaves, on being placed in it, are liable to 

 shrink and twist, thus losing their characteristic 

 shape. No such effect is, however, produced by 

 the use of formalin. The modus operandi (for 

 which I am indebted to the kindness of a friend) 

 is as follows : A shallow cell, of some spirit 

 varnish, must first be made on an ordinary glass 

 slip by means of the turn-table^ the Brown- 

 cement, mentioned on page 183, will answer this 

 purpose. When the cell is quite dry, the slip is 

 again placed on the turn-table ; and the top of the 

 cell is ringed round with marine glue dissolved in 

 benzol (see page 182). The formalin solution is 

 next run into the cell from a glass pipette, until it 



