OF THE DESMIDIACE.E AND SIMILAR MINUTE ALG^E. 57 



which completely supersedes the various unsatisfactory 

 plans hitherto adopted. 



His method differs from all others in this, that he ad- 

 vocates a gradual, not a sudden, application of the preser- 

 vative fluid, so that the action of endosmose may be in 

 some measure retarded, the adjustment of the difference of 

 density within and without the vegetable cell carried on 

 more slowly, and the consequent preservation of its 

 delicate structure ensured. For this purpose he recom- 

 mends the following composition r — 3 parts of alcohol 

 (as pure as possible), 2 parts of distilled water, and 1 part 

 of glycerine. The specific gravity of this mixture being 

 nearly the same as that of water, it does not tend to con- 

 tract the primordial utricle. Now, if the water and the 

 alcohol be allowed to evaporate slowly, the mixture will 

 of course become proportionally denser, but quite gradually, 

 and therefore without any destructive influence on the 

 object. During this operation, the water is withdrawn from 

 the frustule, and the glycerine, which is not volatile, takes 

 its place, without causing any distortion whatever in the 

 plant. 



To make a preparation, the Algae should be laid on an 

 ordinary slide, on which a ring* of the requisite size has 

 been previously drawn with asphalt, the latter being 

 allowed to dry quite hard before being used. To make 

 assurance doubly sure, a drop of distilled water may be 

 laid on the frustule, and then a drop of Hantzsch's fluid. 

 Should the latter, on examining the specimen, appear to be 

 acting too rapidly, water must be added to thin it. With 

 regard to * this part of the operation it is impossible to lay 

 down specific rules, because the Algae, though all very 



* These rings (if round) may be easily and quickly made, and 

 the cement afterwards applied by means of a Shadbolt's turn-table. 

 Square spaces may be enclosed of equal size on any number of slides 

 by drawing a square of the required size on a card, placing the slide 

 over it, and painting it with cement. — Ed. 



