498 



PHYSICAL GEOGRAPHY. 



the water has been driven off without the formation of bubbles, th? 

 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 bum off, and 

 they will become quite white. If, however, there seems to be any diffi- 

 culty 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 dia- 

 toms. 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 sur- 

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

 be laid upon an ordinary glass slide, and examined to see if it is worth 

 preserving, which may be done in one or two ways: first, the glass 

 cover is warmed, and a drop of good spirits of turpentine let fall upon 

 it, covering the diatoms. Just before the spirits evaporate, a small drop 

 of thin Canada balsam is added, and a slide taken, warmed, and a drop 

 of balsam placed upon the centre part of it. Then the cover is brought 

 down upon the slide, the two balsam-covered sides together, in such a 

 way, by tilting the cover slightly, that no air is allowed to come be- 

 tween them, and the cover permitted to fall gradually into place, driv- 

 ing a wave of balsam before it. In this way we have the filamentous 

 diatoms arranged as they grow, but with endochrome removed which 

 would obscure the markings, and in balsam, which renders them trans- 

 parent. Some forms, as some of the Fragillarice, become too transparent 

 if put up in this way, and therefore another method of mounting must 

 be adopted with them. They are burned upon the cover, as just de- 

 scribed, but mounted dry in air; that is to say, a cell of gold-size is 

 made, the glass cover slightly warmed, and then placed upon the cell, 

 with the side upon which the diatoms are fixed, downwards. The 

 warmth slightly softens the gold-size, and the cover becomes fixed. 



Other forms besides the filamentous species may be mounted in fluid, 

 or burned upon the cover and subsequently put up in balsam, or dry. 

 But the commonest way of treating such forms is to clean them by means 

 of chemicals, as already described, and then previous to mounting them 



