■OF THE DESMIDIACEAS AND SIMILAR MINUTE ALGJE. 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: — 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 During this operation, the water is withdrawn from 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 Algse 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. 
