IN FRESH- WATER ALG^E 105 



examined similarly to the two above, and yielded 1-80 mgrms. per 

 litre. There is thus a distinct and quite unmistakable gain of 

 3-05 nigrms., which has no other conceivable source than the atmo- 

 sphere. It may be pointed out that this weight of nitrogen roughly 

 corresponds to about 100 mgrms. of dried alga?, or to 500 mgrms., 

 or about J grm , of moist-plant. It is an increase in nitrogen, 

 lying many times outside the limits of error of the analytical methods 

 employed. 



Experiment II. Commenced May 31, 1918; terminated Sep- 

 tember 9, 1918. This experiment was carried out in a series of 

 twelve screw- stoppered, fruit- preserving jars, called " Kilners," 

 each of about 850 c.c. capacity. These jars are of pale green bottle- 

 glass, and possess a flat flange at top, over which is placed a flat 

 rubber band; on this is placed the glass cover, provided with a flat 

 flange to come in apposition. An air-tight union is obtained by 

 means of a metal screw- top working on a glass screw-thread moulded 

 on the outside of the wide neck of the jar. It was hence possible 

 to keep certain of the jars in connection with the atmosphere by 

 merely covering with a tied-on cap of lawn, and in others to screw 

 down the lids and shut off access to the atmosphere. Each jar re- 

 ceived 200 c.c. of tap water, the supply being Rickmansworth water. 

 This is a surface water containing a small amount of calcium salts, 

 but otherwise a pure, good water. 



Each jar was next inseminated with 2 c.c. of a dilute stirred-up 

 suspension in water of an algal growth which had been developed 

 by self- growth in a small muslin- covered jar on the west window. 

 The amount of nitrogen introduced in this insemination was in- 

 finitesimal, and, in any case, its amount, as well as any trace in the 

 200 c.c. of tap water, can be accounted for by deducting the average 

 small amounts of nitrogen found in Nos. 1, 3, and 4, which really 

 serve as controls; the net increases in the others then show the 

 favouring action on nitrogen fixation of the various procedures. 

 After the addition of the 200 c.c. of tap water and the 2 c.c. of algal 

 suspension in all twelve cases, each was next treated as noted in the 

 schedule below, and afterwards they were either kept closed with 

 glass lids screwed down, or covered with fine lawn only, and were 

 kept in light or darkness as noted. Darkness was secured in a 

 wooden shed close alongside. Light meant full exposure to day- 

 light, on the middle of the grass lawn, except at midday in the hot 

 weather, when a muslin shade was thrown over, and tin sheets 



