82 ALG^E. 



however, that some observers have regarded them as 

 cells, and others as elevations or tubercles on the 

 surface of the valves. 



The preparation of the valves for showing the 

 markings should be effected by burning the frustules, 

 or the mass containing them, on a strip of platinum- 

 foil over a spirit-lamp. The incinerated mass should 

 then be transferred to a slide, and the valves separated 

 with the greatest care by a bristle mounted in a hair- 

 pencil stick under a low power of the microscope. 



This is, however, a substitute for the proper method, 

 which is dangerous in the hands of one unpractised 

 in chemical manipulation. It is this : The mass of 

 Diatomacese (the water containing it having been 

 carefully poured off as far as possible) is put into a 

 Florence oil-flask, and strong nitric acid (aquafortis) 

 gently added, more than sufficient to cover it. The 

 mixture is then carefully boiled over a spirit-lamp 

 for some time. When it is cold, distilled water is 

 added, the whole shaken, and allowed to settle. The 

 watery part is then gently poured off, more water 

 added, and this poured off after settling, and the pro- 

 cess repeated until a drop of the water evaporated to 

 dryness on a slide leaves no residue. The Diato- 

 maeese then form a white sediment at the bottom of 

 the water, and can be transferred to a slide with a 

 dipping-tube. The drop is then dried with a gentle 

 heat, and the valves mounted as dry transparent 

 objects (p. 12). 



If the valves have coarse markings, they may be 

 mounted in balsam; but if the markings are fine, 

 balsam makes them much more difficult of detection. 



Many of the most beautiful Diatomacese are found 

 in the fossil state ; and specimens of these are sold 

 already mounted. I would advise those unacquainted 

 with them to purchase a slide of the ' ' Bermuda " or 

 " Richmond " earth, which abounds in the species 

 of Coscinodiscus ; and of the " San Fiore deposit," 



