38 ON THE CULTIVATION OF DIATOMS 



be greatly modified^ according as it is desired to hasten the 

 multiplication of such and such diatoms. 



Some fresh-water diatoms require for their maximum develop- 

 ment increased proportions of the salts. I have known some that 

 will very well take 40 grains of chloride of sodium to the litre — 

 others 10 to 15 grains of nitrate of soda — many develop strongly 

 under the action of powerful disinfectants. I cannot, in the short 

 exposition that I propose to make here, enter into these different 

 questions, and on these points I would refer the reader to articles 

 that have already appeared, on the Physiology of Diatoms, in the 

 Annale de Micrographie. 



The ascertaining what organic substances are suitable for the 

 culture of diatoms has required a great number of experiments — 

 thus in my early experiments I tried leaves, twigs, and roots of a 

 great variety of plants, and vegetable and animal tissues of every 

 class ; and it was by noting at each experiment the results as more 

 or less favourable that I was compelled to eliminate the greater 

 part of these substances, as sugars, gums, starches, the albumen of 

 eggs and of blood, green plants, etc., and to retain only a small 

 group of organic matters— as the bran of wheat, rye, and oats, 

 the stalks of grasses, mosses, the dried excrement of the rodentia 

 and herbivora, flesh (muscle), washed and cooked, and this last 

 I was afterwards obliged to reject as it favoured the development 

 of fungi and green algae. 



Finally, it is necessary to supply to the diatoms such substances 

 as putrefy slowly and with difficulty, and I may add in quantity so 

 small that the water in which they are immersed shall never at any 

 moment, and especially at the beginning, show that phenomenon 

 of active putrefaction which is indicated by the cloudiness of the 

 mixture under the influence of bacteria. 



A formula for culture which always gives good results is as 

 follows : — Water 1,000, wheat bran 30 to 40 grains, with the 

 addition of i decigramme of wheat straw and as much of moss; its 

 nutritive power will be greatly increased by being mineralised in 

 the way we have already indicated. 



For cultures of less bulk than 300 cms. it is better to use 

 powder bottles — that is, those with wide mouths which can be closed 

 tightly with wadding to preserve the liquid from dust, insects, etc. 



