522 SUMMAJIY OF CURRENT RESEARCHES RELATING TO 



liming as usual, the difficulty of crystallization with the piperine is got 

 over, and also the varnished appearance to the diatom, giving a resolution 

 better than any previous medium. The valves may bo seen with the 

 central rib jet-black, striee a greenish steel-grey, hard and crisp, the outer 

 edge either black or yellow tinged, according to the amount of film the 

 diatom is lying in. 



The author, who discards ringing, states that he is "prepared to 

 mount, clean, label, and resolve the A. pellucida under five minutes' time, 

 in one of the high refractive media, and in no part of the world can the 

 same feat be performed at the present time, so far as our micro informa- 

 tion is to hand to date " (November 1885). 



New preparation of the medium of high index (2 '4) and note on 

 Liquidambar.* — Dr. H. L. Smith's yellow medium consists, as previously 

 noted, of realgar dissolved in bromide of arsenic. It is not, however, the 

 product known in commerce as realgar, that is a brownish-yellow opaque 

 substance with a vitreous fracture, but the realgar of mineralogists, of a 

 beautiful reddish-yellow colour and perfectly transparent. When Dr. 

 Smith published the formula of his medium, the realgar was produced by 

 melting two parts of sulphur with one part of metallic arsenic and keeping 

 the fused mass at a red heat for several hours. After several attempts at 

 making realgar, Dr. H. van Heurck found that it could be more easily and 

 Batisfactorily produced by melting together one part of sulphur and 1 • 7 part 

 arsenious acid in a retort, and raising the temperature to distillation point, 

 Kealgar thus obtained by distillation quite resembles the mineral variety. 

 It is then dissolved, by heat in a test-tube, in tribromate of arsenic, also 

 obtained by distillation. The product is a syrupy liquid of a gi-eenish- 

 yellow colour, almost black in large quantity. 



The diatoms being fixed to the cover-glass by desiccation, are covered 

 with a drop of the liquid medium. The cover-glass is then placed on the 

 slide, and the latter strongly heated in the flame of a spirit-lamp. Large 

 bubbles are given ofi" and the medium assumes a deep red hue, while at the 

 same time the bromide of arsenic volatilizes. When the ebullition and the 

 volatilization are nearly ended, the heating is ceased, slight pressure is 

 applied to the slide, and it is then allowed to cool slowly. As it cools 

 the medium loses its red colour and finally becomes of a pale yellow hue. 

 During the manipulation, which is not difficult in itself, care must be taken 

 to avoid the dangerous vapours. 



Prepared in the manner indicated above, the medium has two disadvan- 

 tages, first, the liquid alters very quickly, and can only be preserved in 

 tubes hermetically sealed, secondly two-thirds of the preparations are spoilt, 

 often very rapidly, and without any apparent cause. In order to remedy 

 these defects. Dr. H. van Heurck made in the past two years numerous 

 experiments, and at last found a method of preparing a solid substance 

 which can be preserved without undergoing change in the air ; and the 

 preparations mounted therein have hitherto kept most perfectly. The 

 author prepares his medium by dissolving in a glass vessel 30 parts by 

 weight of flowers of sulphur in 10 parts of bromine, and thus obtains a 

 solution of sulphur in the bromide of sulphur (S2Br2). After perfect 

 combination, 13 parts of metallic arsenic in impalpable powder are added, 

 and the mass heated until the arsenic is perfectly dissolved. The mass is 

 then poured into a porcelain dish and heated over an open fire and con- 

 stantly stirred with a glass rod until it is found that a small drop is very 

 brittle when cool. The medium is then poured into a cold plate, and when 



* Bull. Soc. Belg. do Micr., xiii. (1886-7) pp. 20-4. 



