462 Miss H. Chick. On a Unicellular Green Alga, [Feb. 28, 



absorbs ammonia with extreme rapidity, and, consequently, will grow 

 and develop to an extraordinary extent in sewage-polluted water, for 

 such water contains a comparatively large amount of ammonia. C. 

 pyrenoidosa also absorbs ammonia in a marked degree, and this fact 

 lends practical importance to the study of its nitrogenous metabolism, 

 for it also may have some bearing upon the purification of sewage. 

 Although the organism is itself very small, and the absolute quantities 

 it tackles also insignificant, yet the results of the following experi- 

 ments, themselves upon a very small scale, tend to show that if C. 

 pyrenoidosa were growing in great quantities {e.g., in the bed of a 

 river polluted with sewage), very important changes might be effected 

 in the composition of the water in which it grew. 



The method of pure cultures was adopted in every case, unless 

 otherwise stated. " Pasteur flasks " were used to contain the cul- 

 tures; these were sterilised and then filled with the sterilised 

 culture fluid by means of sterilised pipettes. One flask was always 

 kept sterile, as a control, while a second was inoculated with 

 Chlorella pyrenoidosa. The flasks were kept, often for months, in a 

 sunny place, protected from dust, and in a damp atmosphere in 

 order to prevent evaporation. These Pasteur flasks can be obtained - 

 to hold 300 c.c, and this size, when containing 150 c.c. of fluid, 

 was found to be extremely convenient. To see if the cultures and 

 controls remained in a sterile condition as regards bacteria, they 

 were tested from time to time by abstracting a few drops and adding 

 them to tubes of ordinary bouillon. These tubes were incubated 

 both at the ordinary temperature and at blood heat, and were 

 watched to see if any bacterial growth took place. The risk of con- 

 tamination was found to be extremely small, and its occurrence very 

 rare. The figures in the following tables refer in every case to 

 cultures that were successfully maintained in a state of purity. It 

 was found, in a few cases, as will be seen in the following tables, that 

 difficulty was experienced in maintaining constant the analysis of the 

 sterile control, when the experiment lasted over a long period. In 

 these cases it would seem as though the ammonia present suffered a 

 slight volatilisation, but this does not vitiate the analysis of the corre- 

 sponding cultures, because the changes in the control are, in all cases, 

 insignificant when compared with the changes taking place in the 

 inoculated fluid. In such cases, it seems a fair procedure to subtract 

 the change in the control from that in the culture, and to consider the 

 difference as due to the activity of the alga. 



Methods of Analysis. — Samples for analysis were removed from the 

 flasks under examination from time to time by means of sterile 

 pipettes. These pipettes were always carefully cleaned, plugged, and 

 then sterilised by heating in a hot-air steriliser at 150° C. for one 

 hour. To protect them from any dust they might attract during this 



