120 Influence of Light on Development of Bacteria. 



ment, but even killed by exposure to the sun's rays, I tried 

 next to discover the time needed for their destruction. 



Exp. V. On 27th March, at 11.30 a.m., four bottles 

 charged with solution, inoculated as in Exp. IV., were 

 taken ; one of them left in ordinary diffused light for a test, 

 and the other three placed in the sun ; and left for 1J, 2 J, 

 and 5 hours respectively, and then put beside the test 

 bottle, the thermometer marking 116°, 124°, and 108° F. at 

 different times in the course of exposure. On the 30th, at 

 9 a.m., the test solution was found to be milky and crusted ; 

 those exposed for 1J and 2J hours showed traces of 

 opalescence, while that which had been exposed for five hours 

 was quite transparent, remaining so till the morning of 1st 

 April, when it began to show slight opalescence ; the others, 

 before that time, having become almost opaque. With the 

 conditions under which I experimented, therefore, five hours 

 proved almost sufficient for the sterilization of the inoculated 

 solution. 



I began now to ask myself in how far the effect, so clearly 

 produced by insolation, might not be due to the solution 

 being raised, by standing on a hot window sill, to a 

 temperature sufficient to paralyse and even kill bacteria, 

 and that independently of any chemical or other action of 

 the sun's rays. The utter want of any such destructive 

 influence in diffused light made this not improbable, and 

 I altered my procedure in the next two experiments. 



Exp.VI. On 6th April,at 2 p.m., the weather being bright but 

 cool, three bottles, containing each two drams of inoculated 

 solution, were suspended outside of a window, in front of the 

 glass, with the same exposure. The 7th was cloudy, the 8th 

 bright and cool, and on the 9th, which was bright and 

 warm, all were still found transparent; and at 9 a.m. one 

 was brought inside out of the sun. On the 10th, which 

 was also bright, another was taken in at 9 a.m., the one 

 which was left out then showing faint signs of cloudiness. A 

 thermometer hung up beside it marked a temperature of 

 98° F. Next clay (the 11th), at 9 a.m., the exposed bottle 

 was quite milky, the others just beginning to show traces 

 of opalescence, the one removed on the 9th being least 

 advanced. Here then the solution which had been longest 

 and continuously exposed to insolation became first altered 

 by bacterial development. There was scarcely any explan- 

 ation conceivable, but that, in all, the development had 

 been retarded by the coolness of the weather at first ; and 



