370 Profs. Percy Frankland and Marshall Ward. 



same time, however, and whiter in colour. In eighty-eight hours the 

 whole of the gelatine was completely liquefied. 



XXII. 



On October 30 about 200 c.c. of sterilised Thames water were 

 strongly infected with the bacillus called B. arborescens by Frank- 

 land, taken from a vigorous culture. 



The infected liquid was divided equally into two Erlenmeyer 

 flasks, one of which was at once wrapped in tin-foil and black paper. 

 The other was supported above a plane mirror, and placed so as to 

 obtain the maximum amount of direct sunshine available from 10 A.M. 

 to 5 P.M. that day ; from 10 to 1 the sunshine was hot and bright, 

 and a control flask showed that the water rose to nearly 20 C., but 

 the afternoon was dull and cloudy, and the temperature fell to 12 C. 

 The temperature in the covered flask, placed side by side with the 

 exposed one, rose and fell so nearly exactly with that of the latter, 

 that no stress can be laid on the difference. 



Before commencing the experiment at 10 A.M., sample plates were 

 made of the infected water ; and at 5 P.M. two plates were made from 

 each flask exposed and unexposed. 



The plates from the freshly infected material showed the usual 

 rapidly growing, loose, thread-like colonies in forty-eight hours, and 

 in three days the gelatine was entirely liquefied to a watery fluid. 



The plates from the flask, wrapped in foil and paper, and sheltered 

 from the direct rays of the sun, behaved similarly, the colonies being 

 a trifle more compact in shape, but equally rapid in liquefying the 

 gelatine completely. 



But the plates from the exposed flask differed from the first 

 onwards from those not insolated. Thus no colonies were visible in 

 forty-eight hours, a period during which the plates from the un- 

 exposed flasks exhibited numerous typical colonies ; the colonies 

 appeared here twenty-four hours later, clearly showing the effects of 

 inhibition due to the light. 



Secondly, when the colonies did make their appearance they were 

 more compact, and instead of shooting out in all directions and 

 covering the plate with a meshwork of fine branches, rapidly lique- 

 fying the gelatine, as in the case of the nnexposed specimens, the 

 mode of growth was so affected that on the fourth day they had 

 developed into beautifully circular yellowish colonies, zoned, and 

 radially striated, and only just softening the gelatine. It was not, 

 indeed, till the sixth day that liquefaction set in generally. 



I explain the differences as follows. The light retards the growth 

 of the living bacilli, owing to some action on their protoplasm which 

 induces interference with the metabolic processes on which growth 



