CULTIVATION OF DIATOMS. 45 



"Among the mineral elements whose fecundating action on diatoms 

 is very remarkable, I shall quote the following salts of sodium, 

 potassium, and calcium, viz. : the chlorides, bromides, iodide, phosphates, 

 and sulphates (i to 5 parts dissolved in 1,000 parts of water, or more 

 concentrated for certain frustular species). Sodium silicate (t in 1,000) 

 would seem to have no effect on these alga?, as they appear to more 

 easily assimilate the silica, contained in vegetable matter and 

 liberated by slow and progressive decomposition, than that of the 

 silica contained in soluble chemical combinations. Lastly, a compara- 

 tively weak solution of the salts of ammonium, particularly the 

 nitrate, impedes the growth of diatoms, but the phosphate of 

 ammonium is an exception and favours it. 



"There is no greater difficulty in producing cultivation of marine than 

 of fresh water diatoms. Artificial salt water which has been obtained by 

 adding to every litre of ordinary water 25 grammes of sodium chloride, 

 1 gr. of magnesium sulphate, 5 gr. of calcium chloride, and - i to - 2 gr. 

 of potassium or sodium, bromide and iodide produces a mineral medium 

 which is found to be very suited to the cultivation of marine Algae, 

 if a few straws and some fragments of Fucus and wrack be added. 



" To obtain absolutely pure cultivations of diatoms the maceration 

 should be prepared without applying heat 8 to 15 days before using 

 and filtered with a Pasteur-Chamberland filter before planting. 



" In the case of ordinary cultivations the planting may be done at once, 

 but the entire maceration must have been previously sterilised at a tempera- 

 ture of 7oc, at which almost all microphytes except Bacteria are destroyed. 



" Diatoms introduced into these various media produce at the end of 

 eight days healthy cultivations consisting of magnificent deposits of a 

 colour varying from golden yellow to dark brownish red and formed 

 almost exclusively of the planted species. 



" By varying the mineral and organic composition of these media, 

 by increasing or diminishing their nutritive power any particular species 

 can be made to predominate. As diatoms are unequally affected by 

 heat and the toxic qualities of antisceptics it is easy by means of this 

 physical and these chemical agents to separate the Algae from one 

 another by making use of a system of diluting. 



"As diatoms cannot withstand the heat of direct solar rays, diatom- 

 cultivations should be placed in a northern aspect either in the open 

 air or behind windows of transparent glass. The light diffused from 

 the sky is always sufficiently powerful to ensure the development of 

 diatoms. Inside poorly-lighted rooms the cultivations are arrested ; but 



