154 THE CULTIVATION OF DIATOMS. 



algae to penetrate the deposit, there to remain, and thence to 

 issue when circumstances require it. 



All these details, which are important, will better find their 

 place in the description of the special arrangements required for 

 the cultivation of each kind of Diatom, for there is only a small 

 group of species that accommodate themselves easily to the same 

 physico-chemical conditions. 



If, for example, we desire to obtain an abundance of the 

 little common Nitzschia, which is found in almost every place, 

 you should add to the maceration some decigrammes of gela- 

 tinous silica, produced from mineral salts, and expose them to a 

 strong light. Under these conditions, there are scarcely any 

 species, but some Cyclostellas and Synedras, which can multiply 

 in the same. If you wish to establish the predominance of 

 Cyclotellas, it will be necessary to add to the liquid 4, 5, or even 

 10 per 1,000 of chloride of sodium ; if, on the contrary, you wish 

 to prevent their multiplication, you should substitute for the 

 chloride of sodium chloride of calcium, and the Nitzschias will 

 become predominant. The Pleurosigma attenuatum fears not the 

 light ; the Pleurosigma sculptum high temperatures ; Pleurosigina 

 angulatum quadratum and Pleurosigma Balticum become superb 

 in yellow light, etc. etc. From this you can see that no one 

 mode of culture is equally applicable to all species. If the nu- 

 tritive substances, organic or mineral, are few, the proportions in 

 which they ought to be employed, according as you wish to obtain 

 a great quantity of this or that species of Diatoms, is very 

 different. 



Thus experimenters need to establish with certainty the 

 chemical composition of the macerations best adapted to the 

 development of those Diatoms that they desire to study, and to 

 determine with exactitude the physical conditions (light and heat) 

 for which they have a marked predilection. This work is con- 

 siderable, long, and delicate, but its utility is undoubted ; for by 

 means of the artificial culture of Diatoms, we shall, I believe, 

 arrive at an elucidation of the many obscure points that surround 

 their history. With a little patience and perseverance, you will 

 be able to cultivate all kinds easily ; for my part, those that I 

 have myself procured, or that I owe to the kindness of M. 



