112 STUDIES OF PHOTO-SYNTHESIS 



supply of carbon dioxide and of nitrites, and in one pair leave the 

 algae destitute of both these ; in the next, supply carbon dioxide and 

 restrict nitrites; in the third, supply nitrites but restrict carbon 

 dioxide; and in the fourth, supply both carbon dioxide and nitrites. 

 The dissociation pressure of carbon dioxide from sodium bicarbonate 

 lies between 1 mm. and 2 mm. of mercury, so it is ample to supply 

 the needs of the algae ; also the nitrous system was so arranged as to 

 give off the nitrous fumes at such a rate that the algae could cope 

 with them and utilise them, so that the nutrient medium did not 

 become acid. 



Examined on August 30 i.e., fourteen days after the addition of 

 the side- tube it was found that Nos. 1 and 2 (no addition) and Nos. 5 

 and 6 (nitrites only) were dead and degenerating. This had occurred 

 in about a day in Nos. 5 and 6, probably because they could not grow 

 from lack of carbon dioxide, and so the oxide of nitrogen absorbed 

 by their culture media accumulated and killed them. Degeneration 

 did not occur in Nos. 1 and 2 for about a week to ten days, but at 

 the end of the fortnight they were obviously dead and degenerating. 



The contrast in the case of the other four jars was striking; all 

 four had lived, increased in amount, and were nourishing at the end. 

 It was obvious that the pair which had received both carbon dioxide 

 and oxides of nitrogen were much in advance of the other pair, which 

 had been supplied with carbon dioxide only. The latter, of course, 

 had a supply of elemental nitrogen in the enclosed air of the jar. 



The united contents of each pair of jars, when Kjeldahled, gave 

 the following results : 



Nxtrogen. 

 Nos. 1 and 2 (no additions) . . . . . . . . 346 mgrms. 



Nos. 3 and 4 (sod. bicarb, only) . . . . . . . . 540 ,, 



Nos. 5 and 6 (oxides of nitrogen only) . . . . . . 2-50 ,, 



Nos. 7 and 8 (both sod. bicarb, and oxides of nitrogen) 14-10 ,, 



The larger weight of nitrogen found in Nos. 5 and 6, which did 

 not grow after addition of oxides of nitrogen, as compared with the 

 controls of the previous experiment, arises from the growth during 

 the first week open to the atmosphere, when the growths were 

 allowed to strike ; no such period was allowed in the preceding ex- 

 periment. If the amount of 2-50 mgrms. in this pair be taken as the 

 control, then the amount of nitrogen fixed by jars with no additions, 

 before succumbing to lack of carbon dioxide, is 0-96 mgrm. When 



