1877.] the Effect of Light on Bacteria fyc. 491 



Our next object was to determine what period of exposure to light 

 might be sufficient to sterilize a given solution. 



Obs. 7. July 10. — Seven full-sized test-tubes were partially filled with 

 solution A and plugged with cotton wool. Six were exposed to the light 

 for periods varying from nine hours to seven days, and were then encased. 

 There was but little direct sunlight upon them during this period, the 

 tube exposed for nine hours having 3^ hours' sun, and the one encased 

 at the end of seven days receiving in the aggregate about 12 hours of 

 direct sunlight. 



All, however, were sterilized, while the seventh tube, which was encased 

 from the first, and served as a control, became cloudy with Bacteria on 

 July 14th. 



Obs. 8. — On July 29th, a very hot day, with much sunshine, six tubes 

 (series a) containing solution A were each inoculated with two drops of 

 a similar solution, which was swarming with Bacteria. They were all 

 then plugged with cotton wool, exposed to the light for periods varying 

 from thirty minutes to eleven hours, and then encased, the duration of 

 sunlight received in each case being carefully noted. 



On July 31st four, which had received half, one, two, and three hours' 

 exposure respectively, were turbid with Bacteria. The other two, how- 

 ever, which had been exposed, the one for five and the other for eleven 

 hours, were clear at this date ; but some days later the former contained 

 some Bacteria and a tuft of mycelium with sporidia, while the latter 

 showed a similar growth of mycelium but no Bacteria. These tubes had 

 received four and a half and nine hours' direct and powerful sunlight 

 respectively. 



On the same day six tubes (series 5) containing the same solution, but 

 not inoculated, were similarly plugged, insolated, and encased. These 

 tubes all contained countless Bacteria on August 2nd. This somewhat 

 dissimilar result we attribute to the fact that for series a we pur- 

 posely chose very narrow tubes, not exceeding a third of an inch in dia- 

 meter, while those of series b were about two thirds of an inch. The 

 fact, however, that, while in Observation 7 we succeeded in sterilizing a 

 solution by an exposure of nine hours, only three and a half hours of 

 which were direct sunlight, nevertheless in Observation 8 (series b) we 

 find the same solution breaking down after eleven hours, nine of which 

 were true insolation, we can only explain by supposing that external con- 

 ditions — notably temperature— may retard or counteract the preservative 

 quality of the solar rays. This point is one which calls for careful in- 

 vestigation. It must be understood, however, that the putrefactive ten- 

 dency of warmth does not in our experience, with this solution at least, 

 override what we have termed the preservative quality of light ; for, pro- 

 vided that there was a full amount of sunlight, we have preserved tubes 

 exposed continuously from day to day as readily in hot weather as in 

 cool. 



