PREPARATION OF DIATOMS. 77 



benzine (not to be confused with petroleum benzine) or, if preferred, in 

 a mixture of sulphuric ether and absolute alcohol, and filtered through 

 a filter paper. The following is the way in which I use the solution 

 thus obtained, whilst I also give a description of the mode in which 

 I have made my preparations for the last ten years or more -the mode 

 indeed in which all my type collections of the Synopsis were prepared : 



I begin by placing the cover-glasses on a large plate of glass, or 

 on a thick piece of cardboard, and on each of the former, by means 

 of a pipette, I put a large drop of distilled water, on which I carefully 

 let fall a small quantity of diatomiferous liquid. (') The diatoms disperse 

 in the drop of distilled water, which is consequently slightly agitated. 



The cover -glasses are then covered over with a bell glass and left 

 to evaporate of their own accord. 



When this is completed the cover-glasses are taken one by one and 

 heated to red heat on a plate of platinum and put back on the 

 large plate of glass where, after having received a drop of very fluid 

 solution of styrax, they are again left under the glass bell to evaporate. 



In a few moments the layer turns white, but this phenomenon need 

 not cause anxiety (it does not occur with the solution in chloroform), 

 and at the end of 24 hours the benzine is completely evaporated. The 

 cover-glass is then turned over and placed on a glass slip and heated 

 gently, preferably with a Marie bath (hot water bath). Having got rid 

 of the air bubbles, if there be any, by means of a clip, it only remains 

 to remove any superfluous styrax from it, when cold. 



Liquidambar is preferable to Styrax. This balsam, which is obtained 

 from the Liquidambar S'yraciflua is not met with commercially in 

 Europe, but it can now be purchased from Messrs. Paul Rousseau & Co., 

 1 7, Rue Soufflot, Paris, purified and hardened according to our method, 

 or in solution (when it is made up either with chloroform or benzine, 

 and is preferable for mounting diatoms). 



Liquidambar is more easily managed than Styrax and has a little 

 higher index of refraction. 



Preparations in very refractive liquids. Various substances possessing 

 a high index of refraction have been proposed for preparing diatoms 

 in, e.g., Monobromide of Naphthalin, Sulphur and Phosphorus dissolved 

 in carbon bisulphide, &c. All these substances are very troublesome to use, 

 and the last two do not keep. They can therefore only be employed tempor- 

 arily and in those cases in which very indistinct details have to be studied. 



( T ) I preserve my diatoms in alcohol, which I decant before using, and substitute distilled 

 water. 



