BY ARTIFICIAL MEANS. 41 



actinic rays are deficient, these algae cannot grow, but they jjreserve 

 for a period of about four months the power of reproduction, when 

 brought to the light of day. During the cold of the last winter I 

 did not observe the development of any of the diatoms of our 

 climate. From about o*^ to 5"C. their rate of multiplication is very 

 small ; it is very appreciable between 5^ and 10^ C, and from that 

 to 30^ C. the temperature is favourable to the greater part of the 

 species. Thus, to have a rapid and prosperous cultivation it is 

 essential that the diatoms want neither heat nor light. These 

 facts, and others well-known to all diatomists, receive here a 

 simple confirmation from direct experiment. 



The eminent diatomist of Belgium, M. le Dr. H. Van Heurck, 

 who has had in his laboratory a spontaneous growth of marine 

 diatoms ever since 1886, and which exists to this day, has 

 remarked that the blue rays are favourable to the life of the 

 diatoms ; his observations are quite true — indeed, two classes of 

 rays are useful to diatoms, the blue and the yellow ; in the red 

 rays the multiplication is insensible. Nevertheless, in my opinion, 

 especially at the commencement of the growth it is well to use 

 coloured glasses, which always produce some obscurity, and which 

 by that means lessen the multiplication of the first frustules that 

 have been sown. All interposition of glass, coloured or uncoloured, 

 of vessels arranged to produce monochromatic light, produce 

 numerous reflections, and offer a very serious loss of the actinic 

 rays, which induces a delay and a slow pace in the multiplication 

 of the algae. Having said so much, the experimenters can easily, 

 as I have myself done, expose the diatoms to white light which 

 has passed through opalised, ground, or fluted glass. 



I have been able also to prove that during the shortest days of 

 the year, in the month of December, you can easily cultivate 

 diatoms, if the place where the preparations are kept is not too 

 cold, in which case a fire should be maintained for several hours, or, 

 better, form a warmed glass case in the window, which could easily 

 be done. 



Under no excuse should the maceration be exposed to direct 

 sunlight, for in our climate, after the month of March, this is 

 sufficiently hot to raise the temperature at times to 45° C, a degree 

 of heat fatal to diatoms. Under the action of the sun's rays the 



