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their virulence. This fact is also of great economic import- 

 ance, as probably the heat of the sun makes most of the 

 germs which would otherwise produce epidemics, more or 

 less harmless. The action of sunlight, specially in the pre- 

 sence of air, in destroying microbes is also recognised. M. 

 Moment of the Pasteur Institute exposed the bacilli of anthrax 

 to sunshine both in the presence and absence of air, with the 

 result that while those exposed to the sun in the presence of 

 air were killed in 2\ hours, those placed in a vacuum were 

 still alive after 55 hours' exposure to sunshine. The rays of 

 the sun that have most effect in destroying microbes are the 

 ultra-violet rays, while the red rays and those nearest to them 

 have little or no effect. Electric light has hardly any potency 

 in this matter of destroying microbes. As heat which is not 

 sufficiently great to kill a microbe is yet able to make it more 

 or less harmless, so is sunshine. Dr. Palermo of Naples 

 showed that comma bacilli, protected from the sun, killed 

 guinea-pigs in 18 hours as usual, but exposed to sun for 3^ to 

 44 hours, they were quite harmless, guinea-pigs inoculated 

 with the bacilli so exposed not developing any kind of disease 

 though it was found that the bacilli were still alive and lived 

 and multiplied as usual. But more than this, the guinea- 

 pigs inoculated with the bacilli thus exposed to sunshine were 

 actually protected from the poison, as after a week these 

 same guinea-pigs were inoculated with the virulent cholera 

 bacilli and they did not succumb. 



1,292. Besides heat, air, and sun-light, there are other 

 potent agents for killing or restraining the growth of mi- 

 crobes. Agents which actually kill microbes are called 

 germicides, eg., corrosive sublimate, quicklime, iron and 

 copper sulphates, chlorine gas and carbolic acid. Agents 

 which only restrain the development of microbes without 

 killing them are called antiseptics, e.g., salt, sugar, oil, 

 and small quantities of sulphate of iron, etc. Germicidal 

 substances used in a very dilute form act like antisep- 



