1877.] the Effect of Light on Bacteria %c. 489 



four were left quite bare. The tops of all were loosely covered with 

 sheet-lead capsules, and the whole were placed in a test-tube-stand out- 

 side a window facing south-east, and about thirty feet above the ground. 

 None of the tubes were plugged. 



May 4. — Tubes examined as to turbidity. The solution remains per- 

 fectly clear in the four bare tubes, while in each of the encased it has 

 become distinctly and uniformly milky. This turbidity was proved mi- 

 croscopically to be caused by innumerable Bacteria. The bare tubes 

 remained quite clear till May 28th, when they were unfortunately acci- 

 dentally lost. 



This observation was again and again repeated with similar result. 



In the large majority of cases the exposed tubes remained clear for an 

 indefinite time, and in every instance were conserved for a distinct period 

 after their encased companions had become turbid. The most marked 

 differences in the conduct of the two sets of tubes were obtained when 

 the sun shone brightly; when for a period of a day or two at the com- 

 mencement of the experiment the weather was close and sultry and the 

 sky dull, the conservative effect of light appeared to be less pronounced. 

 Thus, in an observation started on June 12, it was found on the 14th 

 that while there was a thick zooglcea and advanced turbidity at the upper 

 part of the solution in the encased tubes, there had already commenced a 

 much slighter but recognizable cloudiness in two tubes filled with the 

 same Pasteur's solution, but freely exposed to the light. This result we 

 attributed to the fact that throughout the whole of the 12th and 13th 

 there was not one ray of direct sunlight, the sky being completely over- 

 cast and the atmosphere remarkably thick and hazy. 



Obs. 2. May 5. — Two of the bare tubes used in Observation 1 were 

 taken and the contents were found to be equally and perfectly clear. 

 One was then encased, the other left as before, and both again replaced 

 on the window-ledge for exposure. 



May 16. — the contents of the encased tube are now distinctly turbid, 

 those of the bare tube being perfectly clear. (The latter remained clear 

 till May 28th, when it was destroyed.) 



From this observation it is evident that the fitness of the cultivation- 

 fluid as a nidus for the development of Bacteria is not impaired by the 

 action of light ; for we find that the contents of a tube, which remain per- 

 fectly clear so long as they are freely exposed to the sun's rays, swarm 

 with Bacteria after being deprived of the access of light. This being so, 

 it becomes important to determine whether light may exert, either directly 

 or indirectly, a destructive influence on Bacteria. The obvious mode of 

 settling this question would be to protect the tube to be insolated from 

 subsequent impregnation, and finally to encase it. If then it remained 

 clear for an indefinite period it might be fairly inferred that the Bacteria 

 had been destroyed, either in their rudimentary condition, or successively 

 as they eaine to maturity. 



