828 SUMMARY or CURRENT RESEARCHES RELATING TO 



light like Canada balsam, the preparation becomes absolutely colour- 

 less. 



The solution should be used as follows : — Place the cover-glasses 

 on a large glass plate, and put on each by a pipette a large drop of 

 distilled water, and on this let fall gently a drop of the liquid con- 

 taining the diatoms. Then cover them with a watch-glass and allow 

 to evaporate spontaneously. When this is done the cover-glasses are 

 separately heated to redness on platinum and transferred to the glass 

 plate, and a drop of a very fluid solution of styras or liquidambar put 

 on them and the watch-glass replaced. In twenty-four hours the 

 benzine is completely evaporated. The cover-glass is then put on 

 the slide and slightly heated, preferably in a water-bath. A light 

 pressure will drive out air-bubbles. 



Preparing Shellac Cement.* — E. Hitchcock gives an easy 

 method of preparing an excellent clear solution of shellac. 



Obtain from a paint-shop a quantity of shellac spirit-varnish, or 

 prepare it by dissolving common shellac in alcohol. It is well to 

 use five or six ounces of the varnish, as there will be considerable 

 shrinkage in volume during the process. Place the varnish in a 

 bottle, which it should not more than two-thirds fill, and add to- it 

 about one-quarter of its volume of naphtha or " petroleum spirit." 

 Put in the cork and shake well, to thoroughly mix the two liquids. 

 Let the mixture stand a few minutes and shake it again, repeating 

 the operation two or three times. Then let the bottle stand un- 

 disturbed for twelve hours, or as much longer as convenient. The 

 naphtha will be found in a layer above the shellac containing the 

 flocculent matter, which, being insoluble in cold alcohol, renders the 

 ordinary solutions of shellac turbid, while the alcoholic solution 

 beneath will be perfectly clear. By means of a siphon, extemporized 

 by a rubber or glass tube, the clear shellac may be drawn off from 

 beneath the naphtha. 



The solution thus obtained vyill be too thin for microscopical use. 

 It should therefore be placed in an evaporating dish and heated very 

 gently — preferably over a water-bath in which the water is not 

 allowed to boil — until it reaches a syrupy consistence. When cold 

 it will be thicker than while warm, and it should be tested by placing 

 a few drops on a cold slide and watching its behaviour. When it 

 seems to be right the solution may be poured into a bottle and about 

 three drops of castor-oil added for every ounce of solution. This 

 causes it to flow smoothly from the brush. 



In practice we have found it advisable to evaporate the solution, 

 as above described, until it is too thick to flow from the brush, 

 and then to thin it with strong alcohol. The reason is that during 

 evaporation the alcohol of the original solution is driven off more 

 rapidly than the water that is associated with it. Therefore, by the 

 time the solution is reduced to one-fourth its original volume the 

 alcohol has become much weaker than it should be, and the cement 



* Amer. Mon. Micr. Journ., v. (1884) pp. 131-2. 



