246 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
acid, 10 ccm. 1 per cent, acetic acid, 25 ccm. 1 per cent, cliromic acid, 55 
ccm. distilled water). Next drop 10 per cent, glycerin directly into tho 
fixative, each drop being allowed to diffuse before adding more. Continue 
adding drop by drop until enough glycerin has been put in to cover the 
specimens when evaporated. The fixative and water should now be allowed 
to evaporate in a watch-glass where a large surface is exposed. The speci- 
mens may now be handled with a needle or knife, and arranged on the 
slide under a dissecting Microscope. A drop of pure glycerin or of 
glycerin-jelly makes a very satisfactory mount. Glycerin-jelly has to 
be used very carefully, but it is the more satisfactory when it can be 
used with success. The method was found especially useful for demon- 
strating swarm- spores and the formation of zygosperms. 
Media for the Study of Diatoms.* — Dr. H. van Heurck, in an 
excellent description of the media used for mounting diatoms, divides 
these media into resinous and chemical. Of the first, three are discussed, 
viz. Canada balsam, now but little used, storax, and liquid amber. 
Storax should be purified by dissolving in chloroform and then filtering 
it. Liquid amber is prepared by heating the raw material in a water- 
bath together with equal parts of benzin and absolute alcohol. The 
solution is filtered and evaporated down until the mass becomes slightly 
brittle at about 10°. The mass is then redissolved in the same solvents. 
The chemical media are monobromide of naphthalin, iodide of methyl, 
and Smith’s arsenical fluid. As monobromide of naphthalin dissolves 
most resinous bodies, the preparation must be closed with strong liquid 
glue. Its refractive index is 1 • 658, and hence its visibility is 22. This 
latter is calculated by taking the difference between the index of the 
silica of the diatoms, 1*43, and the refractive index of the medium. 
Iodide of methyl has a refractive index of 1*743, and therefore a 
visibility of 31. If saturated with sulphur its index is raised to 1*787. 
Prof. H. L. Smith’s arsenical medium has an index of 2*4, and con- 
sists of realgar dissolved in bromide of arsenic. This medium is prepared 
by melting together in a retort 1 part of sulphur and 1 * 7 parts of arsenious 
acid, and raising the temperature until the product distils. The realgar 
thus obtained is, together with some tribromide of arsenic, also obtained 
by distillation, heated in a test-tube, the resulting liquid being of a 
syrupy consistence and of a greenish-yellow hue. The diatoms, fixed to 
the cover-glass, are covered with a drop of the medium and then placed 
on the slide. Tho slide is then heated over a spirit-lamp until the 
bromide of arsenic is volatilised. When the gas bubbles cease to be given 
off, the heating is stopped and the preparation allowed to cool slowly. 
During the heating the colour of the medium is red, but as the slide cools 
it becomes yellow. 
This method is subject to two grave objections *. the medium rapidly 
deteriorates even in sealed tubes, and two-thirds of the preparations spoil 
without any obvious reason. The best way to preserve the preparations 
is to make them when the air is dry and the weather sunny. The slides 
and cover-glasses should be warm when the medium is applied. The 
manipulation should be quick and the cover-glass ringed round at once. 
The slides should be kept in a dry place. 
Zeitschr. f. angew. Mikr., iii. (1898) pp. 285-97. 
