30 DIATOMA( 



described in the books treating of the preparation of micro- 

 scopic objects. The vessel in which the fresh specimens of 

 diatomacea k are put up are what are known to microscopists as 

 " cells," but how these are made cannot be gone into here, as 

 the description would occupy too much space and time. Suffice 

 it to say that I prefer cells made of old japan gold size, which 

 can be procured of dealers in microscopic materials. Within 

 such a cell, of sufficient depth and immersed in the preservative 

 fluid, a few of the diatoms, or a scrap of the plant upon which 

 they are growing, is placed, and the glass cover fixed over it in 

 the manner described in the books upon manipulation. The 

 filamentous forms are thus preserved almost in their natural 

 condition; but, on account of the presence of the endochrome, 

 the sculpturing of the siliceous cell- wall is almost invisible. To 

 show this character, while the filamentous form is preserved, 

 another method of mounting is employed. A thin, clean cov- 

 ering glass is selected, and laid upon a clean piece of paper. 

 A large drop of distilled water is then allowed to fall upon it, 

 and in this drop the filamentous diatom is thinly spread out. 

 Then the cover is taken up by means of a pair of forceps, and 

 held over the flame of a spirit lamp, which has been turned 

 down so as to be quite small and steady. The cover is held 

 some distance above the flame, and judiciously manipulated, so 

 that the heat is evenly distributed over it, and it does not 

 crack. As soon as all the water has been driven off without the 

 formation of bubbles, the glass is brought gradually down 

 almost in contact with the flame, and held at that point for a 

 few minutes. Then the diatoms will be seen to turn black, on 

 account of the charring of the organic matter contained in 

 them. After a while this black carbonaceous matter will burn 

 off, and they will become quite white. If, however, there seems 

 to be any difficulty in burning off* the last portions of carbon, 

 the cover is lowered once or twice to come in contact with the 

 top of the flame, and then raised again. In this way it will 

 become red hot for a moment; and everything will be burned 

 off except the siliceous portions of the diatoms. Now the cover 

 is removed slowly from over the flame, and held in the forceps 

 until it is cold, but by no means laid down upon any surface 

 until it is quite cold, — otherwise it will fly into pieces. Then 



