THE INFLUENCE OF LIGHT ON LIFE. 251 



bacilli which were exposed to the sun in vacuum were still alive 

 after fifty hours' exposure. 



A very striking experiment is quoted by Frankland in illustra- 

 tion of this power of the sun. A glass tray is filled with a jelly, 

 in which are mixed the germs of, say, Typhoid bacillus. On the 

 back of the tray is pasted a cover of black paper, out of which 

 has been cut the word " Typhoid." The glass tray is then exposed, 

 upside down, to the sun ; that is, so that the jelly will receive the 

 sun's rays only through the spaces where the letters have been cut 

 out. After being exposed to the sun for two or three hours, the 

 glass tray is put into a dark cupboard and kept at such a tempera- 

 ture as will permit of the germs in the jelly developing and multi- 

 plying. Now, it is manifest that, if the sun has killed the germs 

 in the places where it shone upon them through the cut-out letters, 

 no development will take place throughout the whole word 

 " typhoid," and it is also clear that the germs will grow in all the 

 other parts of the jelly which were screened from the sun by the 

 black paper. This, according to Frankland, is what takes place, 

 the word " typhoid " being quite free from bacterial growths, while 

 the other parts are thickly populated. 



In addition to this killing, or bactericidal, power, as it is called, 

 it has been found that, if the exposure to the light is not sufficient 

 to destroy the microbes, it may greatly alter their character. For 

 instance, in the case of a microbe which produced red pigment, 

 the sun's influence changed its nature so as to prevent it from any 

 longer producing the red pigment ; and, further, Dr, Palermo, of 

 Naples, placed some cholera bacilli in the sunshine, and found 

 that when they had been sunned for three or four hours they were 

 perfectly harmless, and that the animals which were inoculated 

 with them suffered no harm. Indeed, it was discovered that 

 guinea pigs which had been inoculated with the sunned and there- 

 fore innocuous bacilli were rendered proof against the virulent or 

 unsunned bacilli, and thus acted as a vaccination. When the 

 bacilli were not ''sunned," they killed guinea pigs in eighteen 

 hours, as is usual. 



Frankland says, in pointing out the fact that in the Thames 

 water the number of micro-organisms was often twenty times as 

 numerous in winter as in summer, that the presence or absence of 



