80 PREPARATION OF DIATOMS. 



or if I have no further use for it in consequence of having made 

 a better one, my cover-glass is as good as new, because I deposit a 

 fresh diatom on the surface which is now uppermost, and as soon as 

 the cover-glass has been turned over a cleaning with alcohol will clear 

 off the specimen which was last selected and which is still adhering 

 to the surface which will now have become the upper surface. I have 

 repeated such an operation several times without having made any 

 mistake. If large cover-glasses are used, or even if on small cover- 

 glasses it is desired to make a coloured ring round a diatom so as 

 to be able to find it again with greater ease, it is only necessary before 

 dropping the imbibing solution, and as soon as the diatoms are fixed, 

 to place the slip bearing the cover on the turn-table, to centre the 

 diatoms under a lens and make a ring with some water colour paints, 

 e.g., Prussian blue ; this ring will not be injured during subsequent opera- 

 tions, since no more water will be used. 



" C. Selection. I dry the diatoms intended for selection either on 

 English sized slips or, what is even more convenient, on German sized slips. 

 In every case it is important to thoroughly eliminate the alcohol in 

 which the gathering has been preserved, replacing it with very pure 

 distilled water, and allowing the drying operation to go on naturally, 

 without having recourse to heat. In this way one can be certain that 

 the diatoms will not reunite in a mass, and will not adhere to the glass. 

 If the water spreads badly over the glass, it should be cleaned with a solution 

 of bichromate of potass, aciduated with sulphuric acid and containing 

 in suspension a little tripoli (common diatomiforous earth ; that of 

 Auvergne serves the purpose very well). 



" The diatoms can be transferred by means of a hair or bristle fixed 

 to a handle, but I prefer to use small wooden handled brushes. When 

 buying them I ascertain, by means of a lens, that one hair is longer 

 than the others ; I use this brush to transport the diatoms and to fix 

 them. If it is desired to change the position of the diatoms without 

 raising them from the surface, where they are found, it is necessary to 

 use a brush, thoroughly cleansed from all grease by chloroform. If 

 on the contrary one wishes to make sure of lifting the diatoms, the 

 bristle should be greased by passing it over the skin, or belter still, by 

 previously brushing it on a slip slightly rubbed over with essence of 

 turpentine, which has not been completely wiped off. To make sure 

 of depositing the diatom on the prepared cover-glass, the latter should 

 be previously moistened with the breath. The same operations must 

 be gone through whenever it is wished to change the position of the 



