OF THE FREE DIATOMACE.E. 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 Diatomacese 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 Diatomaceas, 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 Oscillatorias 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 



