58 OF THE DESMIDIACEJE AND SIMILAR MINUTE ALGA2. 
sensitive to endosmose and exosmose, are so in different 
degrees. 
Tlie slide must now be laid aside for awhile, covered 
with a bell glass, wine glass, or something of the kind, to 
keep off the dust, while the volatile portion of the mixture 
is evaporating. On its being ascertained that the glycerine 
alone remains, another drop of the liquid is to be applied, 
and a second evaporation submitted to ; and so, again and 
again, until there is enough of the pure glycerine present 
to fill the greater part, though not the whole, of the ring. 
The time has now arrived for covering the object with 
thin glass. Let a piece of suitable size be selected — -the 
thinner the better ; if it be too thick, the chance of ren- 
dering the cell air-tight is lessened ; besides, a thick cover- 
ing glass precludes the use of the highest powers of the 
microscope. Care must be taken to clean it thoroughly, 
just before laying it on, by washing in alcohol or dilute 
liquor potassas, and drying with a soft cloth and leather. 
Nor must it again be touched with the fingers, otherwise, 
at every point of contact, a mark is left, to which the ce- 
ment adheres with difficulty. 
The operator now seizes the thin glass cover with pincers, 
lays a narrow strip of asphalt cement on one of its edges, 
for about one- third of its circumference, and lays it, ce- 
ment downwards, on the object. The glycerine forthwith 
spreads out, and occupies the enclosed space. A gentle 
pressure with the end of the pincers tends to fix the cover 
in its place, and to force out all superfluous liquid, which 
must be absorbed to the last atom by blotting paper, or a 
soft cotton cloth. The best way (though of course nothing 
but long practice can ensure it) is to apply, in the first in- 
stance, only so much glycerine as will fill the shallow cell 
without overflowing. 
The slide must now be put away without being touched 
for twenty-four hours, at the end of which time the strip of 
cement will have dried, and will hold the glass cover in its 
place. The work may, therefore, be proceeded with. The 
