engelmann's researches. 315 



tiveness' of certain bacteria to tlie presence of free oxj-gen. By 

 an ingenious device, simple in its application, it is possible to 

 determine the parts of tiie spectrum in which an assimilating cell 

 or filament gives off oxygen most copiouslj-. Under the stage 

 of the microscope is placed a microspectroscopc, which throws 

 a clear spectrum upon an}- object on the glass slide in its place 

 on the stage, for instance a filament of an alga. The alga is 

 placed upon the slide in water which contains numbers of the 

 common Bacterium (B. Termo), easily procured from putrescent 

 matters. If it is kept from the light, or is exposed to only very 

 faint light, all assimilative activity is suspended, and the bacteria 

 after a time are quiescent. But when light in sufficient amount 

 is permitted to pass through the specimen, assimilative activity is 

 at once manifested, and the evolution of oxygen from the filament 

 brings the bacteria into rapid movement. If, instead of white 

 light, the rays from the spectroscope are passed through the 

 specimen, the activity of the bacteria is equally manifest, but it 

 is confined to a comparatively small part of the spectrum ; the 

 bacteria collecting chiefly at the points whicli are known to coin- 

 cide with the absorption-bands of chlorophyll.^ When a some- 

 what thick cell is employed, there is a noticeable difference 

 between the amount of activity' on its upper and under side. 

 The figures show the ratio of activitj- of assimilation between 

 tiie under side first exposed, and the upper side which receives 

 light that has first passed through a green film. 



It is to be noted tliat Engelmann did not in any case find any 

 assimilation in uncolored chlorophyll, even when the light was 

 tempered by the interposition of a colored medium (compare 850).' 

 He has proved that assimilation proper takes place onlj' in 



^ AccoriUng to Engelmann, the sensitiveness of bacteria is so great that by 

 their reaction the trllUonth part of a milligram of oxygen can he detected 

 (Botanische Zeltung, 1883, p. 4). Clerk Maxwell's estimate of the weight of 

 a molecule of oxygen was one thirteen trillionth of a milligram (Philoso])hical 

 Magazine, 1873, p. 453). 



2 It is interesting to compare these determinations of the point of greatest 

 assimilative efficiency in the spectrum with the results of Langley's researches 

 upon the distribution of energy in the spectrum (American Journal of Science, 

 XXV., 1883, p. 169). 



3 Botanische Zeitung, 1882, p. 419; 1883, p. 17. 



