BY ARTIFICIAL MEANS. 39 



Cultures of greater bulk are more conveniently managed in pre- 

 cipitating glasses or in large glass vessels ; the access of air to the 

 surface of the liquid is necessary, not to furnish the nourishing 

 elements to the diatoms, but chiefly to promote the diffusion of 

 certain poisonous gases, notably, sulphuretted hydrogen, which is 

 always produced in these cultural operations, and which the 

 oxygen of the air transforms partly into water, sulphur, and 

 sulphuric acid. 



Having said so much, each experimenter can vary at will and 

 perfect the formula I have given. My only object has been to 

 publish, here, some general directions, by which it shall be 

 possible, at once, to grow diatoms successfully. 



As diatoms, like the majority of vegetables, take in nutriment 

 by endosmose, and part with the residuary secretion by exosmose, 

 the observer may, if he choose, dispense with the nutritive solids 

 of which I have spoken ; then he should prepare the macerations 

 separately, and ultimately should sow the diatoms in the clear and 

 limpid liquid resulting from an infusion made without heat, and 

 continued for two or three weeks. But those media that spoil 

 rapidly are the best adapted for the culture of diatoms in a state 

 of absolute purity, in which case they must be filtered and placed 

 in vessels, sterilised by being exposed to a temperature of from 

 iio^ to 150^. For full details of the preparation of these nutrient 

 liquids, without the aid of heat, consult my recorded investigations. 

 Finally, as the macerations in which diatoms can be grown are 

 also suitable, in degree, to the multiplication of chlorophytes — 

 especially DesmidiacecE — it is well to avoid as much as possible 

 sowing these green algae. 



2. — Sowmg the kinds of Diatoms. This operation, which in 

 ordinary cases is very simple, consists in introducing into the 

 macerations some of the diatoms that you desire to cultivate. It 

 is essential to point out here that the liquid must be sterilised as 

 regards the green algae, and such diatoms as may possibly be 

 existing in the liquid after its preparation. 



You can, with certainty, sterilise these liquids by placing them 

 in a water-bath, kept for about half-an-hour at 70° C, taking care 

 to ascertain that all parts of the liquid are raised to this temper- 

 ature. In practice it is convenient to use a tin or zinc vessel half 



