40 ON THE CULTIVATION OF DIATOMS 



full of warm water, in which the flasks to be sterilised are plunged 

 to a point above the surface of the contained liquid. 



In the ordinary culture I recommend 70*^ C. as always sufficient, 

 because diatoms perish at temperatures above 45*^ C, and not to 

 boil the liquid for two reasons ; first, to avoid loading the Hquid 

 with too great a quantity of organic matter unfavourably modified 

 by heat ; and secondly, to avoid the precipitation of lime, which is 

 soluble in water as a bi-carbonate, and which boiling changes to a 

 neutral carbonate, almost insoluble in water. 



The diatoms that you wish to multiply should never be sown 

 in a state of dryness, for a desiccation of only a few minutes is 

 enough to kill irrevocably those living frustules that are most 

 charged with endochrome ; possibly the spores of diatoms, if such 

 exist, possess, like those of mosses and bacteria, the power of sur- 

 viving a long term of drought, but this has yet to be established. 

 In consequence, it is necessary always to introduce into these 

 macerations, either diatoms held in suspension in water or the 

 same algae contained in moist receptacles, or such as are obtained 

 by allowing the water to run away very slowly. 



As to the sowing — that is, the introduction of the diatoms into 

 the nutrient liquid^ — you can use the point of a pipette, previously 

 made hot, or a wire of platinum, having its end flattened so as to 

 act as a small spoon. The pipette, with a point finely drawn out, 

 will meet most of your wants. 



If you need to sow a single living frustule, the operation 

 becomes much more delicate. It requires then the application of 

 special, well-known methods — of fractional sowings — of Hansen's 

 process — and of other methods that I shall describe later on in 

 the second chapter of this memoir, devoted to the cultivation of 

 diatoms in a state of purity, in which I shall show that you may 

 simplify this operation by a first sorting of the diatoms by heat — 

 by antiseptics — by nutritive media greatly modified — in a word, by 

 all that tends to give a preponderance to the species you desire to 

 isolate. 



3. — Of the manage7?ient of the Culture. The sowing being 

 made the maceration should be exposed to the north, either in 

 the open air or in the house behind a glazed window. Diatoms 

 will not develop in the dark, nor in a half light. When the 



