BY ARTIFICIAL MEANS. 153 



For lighting the growths a bat's-wing burner, or a circular 

 burner, may be used, entering the chamber by a central aperture, 

 and of which the supply of gas is regulated by a pressure guage, 

 and the quantity burnt per day being ascertained by a small 

 meter. You can also place between the growths and the light 

 cylinders of glass, coloured or ground, so as to diminish the 

 intensity of the luminous and heating rays ; rendering the flame 

 of the gas less flickering, more brilliant, and modifying the nature 

 of its radiations. The electric light, less heating and more easy 

 to manage, might certainly be substituted for gas-light with advan- 

 tage, but I have not been able to employ it in my experiments. 



In order not to confuse the description of the simple facts 

 that I have noticed, I have not spoken of the many modes of 

 growth that the operator may attempt. Not only is it possible in 

 certain cases to employ glass vessels, either plain or coloured, 

 stoneware, or earthenware, but to carry on the cultivation in 

 continuous currents of water, provided that the water employed 

 to renew that which the Diatoms have exhausted for their nutri- 

 tion shall arrive slowly, and after having been perfectly filtered. 

 To carry out this arrangement, the filtered water should be con- 

 ducted to the centre of the liquid, and delivered, drop by drop, 

 from a glass tube, or a fine thread, whilst the excess is allowed to 

 escape at a lateral tubule at the top of the glass. 



You may also introduce into the nutritive liquids some sub- 

 stratum on which certain Diatoms love to fix themselves. Twigs 

 or bits of wood that have been previously boiled, or macerated 

 for a long time previously ; fragments of marble, of earthenware, 

 of chalk, of various rocks, oyster-shells, flint nodules, etc. The 

 introduction into the growths of these various substances, that 

 are more or less indestructible, generally without action on the 

 development of the Diatoms, permits them to seek and choose 

 for themselves the spot most favourable to their multiplication. 

 To cultivate some very fragile kinds I have prepared deposits of 

 artificial flocculi, in the middle of which many Diatoms find the 

 degree of light that is most favourable to them. These flocculi 

 are composed of the silicate of magnesia, hydrate of allumina, or 

 of pure hydrate of silica. The two first substances form floccu- 

 lous clouds, of which the extremely slow aggregation permits the 



