OF THE FREE DIATOMACE2E. 
19 
reddish hue ; a proof that not an atom of the acid is left in 
the glass. 
The greatest attention must be paid to this cleansing 
process, for, should it be carried out carelessly or incom- 
pletely, the valves, on account of the acid still clinging to 
them, will make very imperfect preparations for the micro- 
scope. Supposing, however, that all has been done as it 
should be, the flinty coating of the Diatomaceas suspended 
in the water will be seen to give it a sort of flickering 
appearance. Time must be given them to settle on the 
bottom of the glass, and then they may be passed through 
the filter and dried ; or, if not wanted immediately, they 
may be kept in alcohol for future observation. 
The species which lie on the surface of mud must also 
go through a process of cleansing, for it is exceedingly 
difficult to collect them in so pure a state as is needed for 
a really good preparation. They can rarely, in fact, be 
gathered without a large infusion of sand and earthy in- 
gredients, though, of course, the more careful the col- 
lector is in taking them up, the less will be his trouble 
and anxiety afterwards. 
Generally speaking, these species appear as a yellowish 
brown deposit at the bottom of the ditch or shallow pond 
in which they are found. The finer the weather and the 
brighter the sun, the richer and more conspicuous is this 
deposit, because these circumstances are favourable to the 
vegetative powers of the Diatomacese, which then move 
with considerable activity, and draw themselves towards 
the light, -forming a thin layer, which may be lifted 
carefully off the surface of the mud with an iron spoon, 
and dropped into a wide-mouthed bottle. Sometimes, 
under the influence of the sun’s rays, causing gases to 
bubble up from the underlying mud, myriads of these tiny 
plants are caught by passing fragments of Oscillatorise and 
other weeds, are raised to the top of the water, and may 
then be secured in a perfectly pure condition. 
When however, as is more usually the case, earth and 
c 2 
