280 CHAPTER XX. 



1 vol. of Price's glycerin. Mix, and add 1 vol. of chloral 

 hydrate (i.e. add crystals of chloral until the volume of the 

 mixture has increased by one half) ; warm till dissolved. 

 This gives a very highly refractive aqueous mounting medium, 

 which is found useful for opaque tissues that it is desired not 

 to dehydrate. 



A similar medium is published by GEOFFKOY, Journ. de 

 Botan., 1893, p. 55 (see Zeit. f. u-iss. Mik., ix, 4, 1893, 

 p. 476). He dissolves, by the aid of as little heat as pos- 

 sible, 3 to 4 grms. of gelatine in 100 c.c. of 10 per cent. 

 aqueous solution of chloral hydrate. 



High Refractive Liquids. 



435. STEPHENSON'S Biniodide of Mercury and Iodide of Potas- 

 sium (Journ. Roy. Mic. Soc. [N. S.], ii, 1882, p. 167). A solu- 

 tion of the two salts in water. " This is very easily prepared 

 by adding the two salts to the water until each shall be in 

 excess ; when this point of saturation has been reached the 

 liquid will be found to have a refractive index of T68, by 

 far the highest of any aqueous solution known to me." 

 Any 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, with no 

 appreciable difference in the refractive index." 



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 

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

 a permanent mounting medium. Tissues are well preserved, 

 but the preparations are ruined by a precipitate which forms 

 in the fluid. But as a temporary examination medium I 

 have occasionally found this solution valuable. Its optical 

 properties are wonderful ; it allows of the examination of 

 watery tissues, without (my dehydration, in a medium of 



