54 CULTIVATION OF DIATOMS 



liquid, suspends or checks the development of the diatoms. Chemical 

 reagents have not the same toxic effect on all forms of diatoms. The 

 following are the results which have been obtained by Dr. Miquel. 



Mercury chloride kills diatoms in doses between i in 30,000 and 1 

 in 40,000. 



Cupric sulphate acts like the former. 



Zinc sulphate in a dose of 1 in 40,000 considerably impedes the de- 

 velopment of diatoms. 



Iron introduced in the form of ferric solutions is rapidly decomposed, 

 and is thrown down as a fiocculent precipitate of yellow hydrated ferric 

 oxide, the presence of which obstructs the development of diatoms if 

 too much is added; in a very weak dose (see solution B.) it may 

 assist the growth of diatoms. 



Arsenious acid acts very toxically on diatoms. It kills most of them 

 even in so weak a dose as 1 in 30.000. 



The Mineral acids in doses strong enough to produce a sensible reaction 

 produce a very harmful effect. 



Iodine produces the same toxic effect as zinc salts. 



Boric acid and Carbolic acid are not prejudicial if weaker than 1 in 

 10,000. 



Essential oils are opposed to the development of diatoms. 



6. Effect of Anesthetics. Chloroform has a powerful toxic effect on 

 diatoms ; the vapours of Bromoform and Iodine also produce very harmful 

 effects on them, but act more slowly. Lastly, the vapours of sulphuric 

 ether and iodoform are less harmful, and carbolic acid has only a temporary 

 effect. 



7. Effect of Indifferent and Nutrient Substances. Dr. Miquel 

 has not yet published his experiments with these substances. 



8. Forced Production of Auxospores in Cultivations. Dr. 

 Miquel, who has studied this phenomenon in connection with Melosira 

 varians and Nitzschia palea has summarised his observations on this 

 subject as follows : 



" 1st. It is now unnecessary to rely on chance observations to 

 observe the formation of auxospores in diatoms ; the experimentalist, by 

 means of successive artificial cultivations of these algce in a pure condition, 

 can most easily produce for himself millions of sporangial frustules of 

 both filamentous and free diatoms. 



" 2ndly. The size of diatoms is as a general rule established indepen- 

 dently of the production of the spores or sporanges. The protoplasm of 

 micro-frustules when near their size-limit increases in volume, escapes from 



