44 OF THE CHARACTERISTICS OF THE DIATOMACEiE. 



quantity of the Diatomaceas on a thin sheet of platinum, and 

 holding it over the name of a spirit lamp. As soon as the 

 foreign matter is consumed, the frustules. are to be removed 

 from the platinum, and spread out on the stage plate for 

 examination. 



It is sometimes more convenient, if the individual plants 

 are very minute and diffused through water, to employ scales 

 of mica in lieu of platinum. But care must be taken in the 

 selection of' the mica, as not every piece that comes to hand 

 is suitable for the purpose. A piece must be chosen, which . 

 separates easily into flakes, is clean and pellucid, and does 

 not grow dim when exposed to extreme heat. Moreover, it 

 must not be too thick, otherwise the scales are apt to fly apart 

 during the process of incineration ; nor too thin, because, in 

 that case, it becomes dull and opaque under the influence 

 of the heat. Experience alone^ can guide the operator in 

 choosing the right thickness. 



When the frustules have been laid in a drop of water on 

 a scale of this description, the latter is to be held, by the 

 aid of a forceps, at some little distance above the flame, so 

 that the moisture may evaporate without boiling. Strict 

 attention must be paid to this point ; for, should the water 

 be allowed to boil, the frustules are thrown on their edges 

 by the action of the air, as it is set free, and long dark 

 heaps are formed on the surface of the mica, like Lilliputian 

 moraines. So soon as the moisture is entirely dissipated, 

 the plate may be lowered into the flame, and there held. 

 At first the preparation assumes a black colour in conse- 

 quence of the organic matter becoming carbonised : in the 

 next stage it turns red, and finally white ; a token that all 

 the foreign bodies have been got rid of, and the empty 

 valves alone remain. 



The period during which the frustules should be exposed 

 to this great heat is a matter of considerable moment, and 

 must be carefully watched. Too short a time is insufficient 

 to destroy the organic substances, and the valves come out 

 dark and impervious to light. If the heating continues too 



