100 



traneous balsam around the cover is now removed with a pen- 

 knife, or a brad-awl, and the slide cleaned with alcohol, and wiped 

 dry. 



B. Filamentous and stipitate species must be mounted by the 

 above process to show the valves, and by another special one to 

 show the genus. They are placed in alcohol, which removes the 

 endochrome without breaking up the chains, and then a small 

 portion is dried on a cover, and when it is cool a drop of turpen- 

 tine added. This permeates the frustules, and prevents the en- 

 trance of air when the balsam is added. A small quantity of 

 balsam, in a fluid state, is now dropped on it and heated very 

 cautiously over the spirit lamp, whose wick has been pushed 

 down until only a small blue cone of flame is visible. A drop of 

 balsam is placed in the centre of a clean and warm slide, and, 

 after that on the cover has been sufiiciently thickened, they are 

 united, pressed together, and allowed to cool, when they may be 

 cleaned as above. This process preserves the filaments, and 

 brings the specimen nearer to the object-glass than if it were 

 dried on the slide. 



C. Sometimes the treatment with alcohol does not remove all 

 the endochrome, when it will have to be burned out over the 

 lamp as follows : The Diatoms are spread on the cover and dried. 

 The cover is still kept over the small blue flame, and we see the 

 specimen turn of a light brown, then darker, until it is black, and 

 finally white, when it is brought down smartly once or twice to 

 the flame. This must not be done too suddenly, nor must the 

 glass be removed too soon, or it will crack from the sudden 

 change of temperature, and if the thin glass be kept too long over 

 or against the flame, it will bend and be useless. 



D. Diatoms which have been dried or burnt on the cover may 

 be mounted dry by means of cells. These are best made of 

 Brunswick black, or some other varnish, and they should be 

 made in quantity and kept on hand. The method of making 

 them is given in most of the handbooks. "When the Diatoms 

 have been dried on the cover, and the glass is still somewhat 

 warm, it is placed upon the ring of varnish, with the Diatoms 

 toward the slide, and pressed down. If it does not at once ad- 

 here, it is not advisable to heat it from below, but a warm knife 

 may be laid on it, or a warm wire passed around on the thin 



