ZOOLOGY AND BOTANY, MICKOSGOPY, ETC. 457 



contents, and they must be lifted again in order to replace it ; other- 

 wise it is as convenient as any other form, while it has the great 

 advantage that any one of its trays may be used at any time as an 

 independent box ; still further, its cost is about one-third of any 

 comparable cabinet. The binder's board trays have, when first made, 

 a little tendency to warp, and had better be kept under pressure, but 

 this is only temporary. 



Mbller's Typen- and Probe-Platten.— The catalogue just issued 

 by Mr. J. D. Moller contains a somewhat startling item — a " type 

 plate" of 1600 arranged diatoms, the price of which is 1600 marks 

 or 80Z. ! With 800 or 400 diatoms, 20Z. and 3L 15s. is asked. 



Mr. Moller also issues type plates of 100 and 400 diatoms with 

 the names of each photographed beneath. 



All the type plates are mounted in monobromide of naphthaline. 



Twenty-four test objects (diatoms) are now issued in eight different 

 forms — viz. in air, balsam, monobromide of naphthaline, and phos- 

 phorus, and with cover-glasses of 0*16-0 '20 mm. or 0-06-0 -08 mm. 

 These include AmpJiipleura pellucida, Frustulia saxonica, Pleurosigma 

 angulatum, and Surirella gemma. The " Probe-platten " of 20 and 

 60 diatoms are also supplied in the four different forms of mounting. 



Slack's Silica Films.* — Mr. H. J. Slack suggests an alternative 

 mode of obtaining the silica deposit to that originally published. 

 The old plan was to mix a teaspoonful of powdered fluor spar and 

 rather less of powdered glass in a wide-mouthed 6-oz. bottle, pouring 

 on it enough sulphuric acid to thoroughly wet it. Then place a 

 loose moist tuft of cotton wool in the mouth of the bottle, put a paper 

 cap over it to check evaporation and leave for some hours, when the 

 cotton will be found to have a deposit of silica upon it like hoar 

 frost. This deposit being scraped off into a watch-glass, and water 

 poured softly on it and run off quickly, pure hydrate of silica is 

 left in various curious shapes, some very much like portions of 

 well-known diatoms. By the modified method, instead of allowino" 

 the silicic-fluoride gas to come into contact with wet cotton, 

 some of it is passed through a mixture of four parts of glycerine 

 and one of water. This is readily managed by using a very 

 small flask or a tube bottle to contain the fluor spar, glass, and 

 acid, and fitting to its mouth a few inches of bent glass 

 tube. A gentle heat from a spirit-lamp causes the gas to be given 

 off freely, and by dipping the tube just under the glycerine and water, 

 which may be held in an egg-cup, silica films are instantly formed. 

 The experimenter must be on the watch lest the tube gets stopped up 

 with the silica deposit. As soon as it shows any signs of this, clear it 

 out with a fine wire. Only a very small quantity is required of the 

 various chemicals — a quarter or less of the quantities in the original 

 experiment. The films should be washed, and then gently crushed 

 and mounted, to be viewed with l-4th and l-8th in. objectives and 

 dark-ground illumination. This is easily managed if the objectives 



* Knowledge, iii. (1883) pp. 82-3. 



