244 CHAI'TKK XIX. 



lower index can be obtained by suitable dilution with water. 

 This fluid is very dense, its specific gravity being 3*02. It 

 is highly antiseptic. 



For marine animals a weak solution is probably well 

 adapted^ as about a 1 per cent, solution (5 minims to the 

 ounce) will give the specific gravity of sea water. 



Covers should be sealed with white wax, and the mounts 

 finished with two or three coatings of gold size and one of 

 shellac. 



I have experimented both with strong and with weak 

 solutions. They are not adapted, I find, for the purposes of 

 a permanent mounting medium, for the preparations are ruined 

 by a precipitate which forms in the fluid. But as a tempo- 

 rary examination medium I have occasionally found this 

 solution valuable. Its optical properties are wonderful ; it 

 allows of the examination of water} 7 tissues, without a-mj 

 dehydration, in a medium of refractive index surpassing that 

 of any known resinous medium. 



See further details in early editions. 



441. Monobromide of Waphthalin. See Journ. Roy. Mic. Soc., 

 1880, p. 1043 (ABBE and VAN HEURCK), and Zool. Anz., 1882, p. 555 

 (MAX FLESCH). 



Resinous Media. 



442. Resins and Balsams. Resins and balsams consist of a 

 vitreous or amorphous substance held in solution by an essen- 

 tial oil. By distillation or drying in the air they lose the 

 essential oil and pass into the solid state. It is these solidi- 

 fied resins that should be employed for microscopical pur- 

 poses ; for the raw resins always contain a certain proportion 

 of water, which makes it difficult to obtain a clear solution 

 with the usual menstrua, is injurious to the optical properties 

 of the medium and to the preservation of stains. All solutions 

 should therefore be made by heating gently the balsam or 

 resin in a stove until it becomes brittle when cold, and then 

 dissolving in an appropriate menstruum. 



Solutions made with volatile menstrua, such as xylol and 

 chloroform, set rapidly, but become' rapidly brittle. Solutions 

 made with non-volatile media, such as turpentine, set much 

 less rapidly, and pass much less rapidly into the brittle state. 



