ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 901 



which lasts for thirty minutes ; some fresh silvering solution is then 

 applied and allowed to act at the same temperature for another thirty 

 minutes ; by this means a somewhat thick coating is given. 



Smith's new High-refractive Media.* — Prof. H. L. Smith writes 

 as follows : — 



" The results of experiments made subsequently to the discovery 

 of the boro-glyceride and antimony bromide medium, described in a 

 preceding paper,! are °^ importance, and demand a brief notice. 

 The antimony compound works very pleasantly, and still appears to 

 be the best when high refractive power is required ; but unless all 

 excess is completely removed from outside the cover, it stains the 

 protecting ring. The litharge and gold size ring and the zinc white 

 ring are merely darkened ; but the black asphalte ring is softened. 

 Thoroughly cleaning off the excess around the cover remedies this 

 difficulty. 



The chief improvement I would make in the formula given, I now 

 think, is the substituting of stannous chloride for antimony bromide, 

 and of arsenious acid for boracic acid. 



I find that a compound of stannous chloride, arsenious acid, and 

 glycerin is so very slightly deliquescent, that the mounts may be left 

 for weeks without cleaning off the excess, and that very little if any 

 softening of the material ensues. The mounts are easily cleaned, as 

 the cover is very firmly attached. 



The medium is not so liable to turn when heat is applied, as when 

 boro-glyceride or gelatin. and glycerin are used; the latter, indeed, 

 for that reason, is quite objectionable. The refractive power of the 

 mixture is not quite so high as when antimony bromide is em- 

 ployed ; but the refractive power is quite high enough for anything 

 except the most hyaline tests ; and as a little excess of material 

 outside the cover does not discolour the ring, and does not seem to 

 alter by quite long standing without a ring, I now prefer this 

 compound. 



This medium is prepared as follows : — Weigh out 6 parts of 

 stannous chloride, and 2 to 2^ parts of pure arsenious acid. Melt 

 the stannous chloride in a test-tube, and boil it for a little while ; 

 add while hot an amount of glycerin equal to the bulk of the melted 

 stannous chloride, not more ; heat and shake until it forms a perfectly 

 clear solution. Add now, little by little, the arsenious acid, con- 

 stantly shaking and heating until all is dissolved. This mixture 

 when cold should be very viscid. 



In making a preparation with this medium, at first, on heating, 

 a great number of small bubbles may appear under the cover. A little 

 more heating enlarges these to steam-bubbles ; then, by allowing the 

 slide to cool a little, the cover will settle down, and most of the 

 bubbles will disappear ; but if any are still present, another applica- 

 tion of the heat of a small flame under the slide at the edge of the 

 cover, where the bubbles are most abundant, will remove them. 



* Journ. Now York Micr. Sue, ii. (1SS0) pp. 75-7. 

 t Sec this Journal, ante, p. 356. 



