150 ON THE CULTIVATION OF DIATOMS 



Part III. — General Directions and Notes on the Growth 



OF Diatoms. 



Despite all the precautions that I have been obliged to insist on, 

 the Diatomist will hug a delusion if he thinks that the cultivation 

 of the frustular algae is always an easy task. In order to avoid 

 the annoyance that failure occasions, the operator ought, in the 

 first place, to make serious study of the different manipulations 

 that the cultivation requires. Then he should understand 

 thoroughly the light that visits his laboratory, and which will often 

 be found to act in very different manners, as the days are long or 

 short. He must take precautions to protect his growths from the 

 excess of the luminous radiations, as well as from the deficiency 

 of these radiations. Some screens, some curtains, some stages, 

 so placed as to admit of the governing of the lighting, which 

 ought to be continually looked after and intelligently managed, 

 will be required. On the dark and rainy days of summer, the 

 growths of the large and beautiful Diatoms should be exposed to 

 the north, and immediately behind the sashes ; if the sun burns, 

 they should be carried into a half-darkened chamber. In winter, 

 constant exposure towards the north offers less diiEificulty, and the 

 oversight is much easier. 



Thus, then, in the majority of cases, we must protect the 

 Diatoms against too much light. If in Nature the Diatoms instinc- 

 tively fly from these luminous rays, that are hurtful to them, and 

 seek those that are helpful in the narrow part of the flask, they 

 are evidently compelled to submit to such physical conditions as 

 the experimenter forces on them ; if these conditions are adverse, 

 the Diatoms will not develop, and this is a frequent cause of 

 failure. On the other hand, if many species are mingled together, 

 those that receive the luminous radiations most congenial to them 

 which develop, will occupy the centre, and smother the adjoining 

 species. You will quickly perceive many kinds of little Nitzschies 

 displaying their pretty frustules, after the manner of weeds ; if 

 they have once got foothold in a maceration, it is difficult to get 

 rid of them, whence the necessity, as I have before said, of 

 establishing for each kind of Diatom a special form of culture, 

 suitable for assuring the predominance, and favouring the multi- 

 plication of the species desired. 



