BY ARTIFICIAL MEANS. 365 



Melosiras, and Cyclotellas ; whilst there are others that accept this 

 feeble hght as a pis aller\ others vegetate little with all the light 

 from gas. These conditions of invariable light do not occur in 

 nature. Thus, I reckon the culture of Diatoms and green Algae 

 by gaslight or under the action of other sources of artificial light 

 as abnormal. 



If, in carrying on the culture of Diatoms under the variable 

 action of daylight you modify the temperature ; if, for example, you 

 raise the temperature to between 30" and 35^ C. — a temperature 

 that running waters in our climate never acquire — you will observe 

 with some species an exceedingly rapid development. The 

 Nitzschia palea^ left at 32^ and 33"^ C. under the action of a bright 

 light, increases so quickly that twelve hours after sowing in a 

 maceration nutrified with straw, its development is easily visible to 

 the naked eye, while many of the cryptogams and the bacteria will 

 not show at all in so short a time in the maceration in which they 

 are sown. The Nitzschia palea^ in thus developing, sets up a very 

 abundant disengagement of oxygen. The bubbles of gas rise from 

 the bottom of the vessel as thickly as the bubbles of carbonic acid 

 from an alcoholic fermentation in good action, and, by adapting a 

 conducting tube to the neck of the flask that contains the macera- 

 tion, covered with froth, you may collect in five or six days more 

 than a litre of oxygen almost pure. Many Diatoms placed under 

 like conditions would never develop. It is found that the tem- 

 perature most favourable to their prompt increase is very low — 

 about from 5^ C. to 6*^ C, a temperature equally abnormal for the 

 Parisian climate. 



In modifying the intensity and nature of the luminous radia- 

 tions — in other words, making them strong or feeble, red, yellow, 

 or blue — you make a change in the speed of the growth of 

 Diatoms, you retard or accelerate, and other things being equal, 

 obtain growths differing greatly from those obtained under white 

 light. These results ought also, in my opinion, to be considered 

 as curious, and perhaps useful, anomalies. 



But where the field of research becomes immensely vast — 

 indeed, indefinite — is when, leaving the domain affected by physical 

 agents, you turn to the innumerable chemicals to impart to the 

 cultures special features altogether distinct from those that you 

 note in the normal media. 



