CH. VI] GENERAL CHARACTERS OF BACTERIA 89 



decided influence on some bacteria, since they grow well in 

 the light, while on others diffuse daylight, and still more 

 decidedly direct sunlight has a strongly deleterious effect. 

 According to Cohn and Mendelssohn,^ strong electric 

 currents have a noxious influence on the growth of- micro- 

 cocci. 



Engelmann ^ describes a bacterium photometricum, the 

 motility of which directly depends on light ; it ceases in 

 the dark. Duclaux found that exposure to direct sunlight 

 injures the hfe and growth of some bacteria, both septic and 

 pathogenic. 



The powerful inhibitory influence which insolation has on 

 the growth and life of aerobic bacteria has been first in- 

 vestigated by Duclaux, then by Downes and Lunt, and 

 more recently by Buchner, Marshall Ward and others. 

 This latter observer was the first to demonstrate the 

 important differences of action that exist in the red and 

 blue end of the spectrum, the latter acting more decidedly 

 bactericidal than the former. Dr. Westbrooke made the 

 important contribution to this subject by showing that this 

 germicidal action of sunlight depends on, or rather comes 

 into play during free supply of oxygen, that this action is 

 absent when oxygen is absent (or for instance in the case of 

 anaerobic microbes which grow only in absence of oxygen) ; 

 he further suggests that in the case of aerobic bacteria the 

 germicidal influence of light may be due to oxydising or 

 ozonising influences. 



Bacteria may be roughly divided after Pasteur into two 

 great groups, according to whether they grow under, and 

 require free access of oxygen — aerobic, or whether they 

 can do and grow better without it — anaerobic. On more 



1 Cohn's Beilr. z. Biol. d. Pfl. Bd. iii. i. 



^ Unters. atis. d. physiol. Labor. Utrecht, 1882. 



