:>9() E. J. ALLKX AND K. W. XELSOX. 



the colour and quautity of diatom growth, and in the case of 

 Thalassiosira decipiens the chains are long and well 

 formed. With animal-charcoal water + solution B, on the 

 other hand, practically no growth was obtained. 



It is possible that a certain amount of phosphate, and 

 perhaps of calcium, from the animal charcoal, goes into 

 solution and serves as a "nutritive^' material for the diatoms. 

 J5nt we are inclined to think that its chief action is "protec- 

 tive," and due to its power of occluding gases, such gases 

 being in a state of higher chemical activity than under 

 normal conditions.^ 



As was explained in a previous section (p. 378), the 

 possibility that the action of animal charcoal might have 

 some sort of effect comparable to oxidation, led us to experi- 

 ment with hydi'ogen peroxide. Fair growths of diatom could 

 be obtained in sea-water prepared in the manner described, 

 but they always showed a tendency to rather rapid exhaustion. 

 As in the case of animal-charcoal water, tank-water proved 

 a much better basis for treatment with H^.O^ than outside 

 water. 



Keviving T^xhausted Cultures. — Several experiments 

 were carried out with water from old, exhausted cultures. 

 The sediment was filtered off, the filtrate was sterilised by 

 heat, and then treated by various methods. 



In one typical experiment the following was the result : — 



Water from an exhausted culture of Skeleton ema 

 costatum in jliquel sea-water, reinoculated with the same 

 diatom : 



A. Filtered and sterilised. 



No growth obtained. 



B. Ditto + solution A (nitrates only). 



Good culture, but did not last very long; farther 

 addition of nitrates made no improvement, 

 c. Ditto -|- solution B. 



' Against this view -would seem to be tlie fact that when powdered 

 oocoa-niit charcoal, which has a still higher power of occluding gases, 

 Avas used in place of animal charcoal, very poor cultures were obtained. 



