1894.] on the Action of Light on Bacteria and Fungi. 263 



of germinating at all — they are directly hilled by some injurious action 

 set up in their interior by certain rays, and this at temperatures much 

 lower than that we are now employing, whereas these spores are not killed 

 by mere boiling for a minute or two, and will withstand for hours, and 

 even days, temperatures far higher than any they are subjected to in the 

 experiments I am going to describe. 



In experiments where the spores of Bacillus anthracis were left 

 in distilled water exposed to light for a few hours, it was found that 

 many of the spores died, while of those which were left the germi- 

 nating power was very much weakened. If, for instance, a number 

 of spores were distributed in pure distilled water, and thoroughly 

 shaken up, and the infected water was then divided into two portions, 

 one exposed to light and the other not, every sample drop taken 

 from the former tube was found to contain far fewer living spores 

 than any sample drop from the latter. This confirmed the results 

 observed by others, and already referred to — the sunlight in some 

 way kills the spores, even in pure water. 



It was further noticed that if 1 mixed a drop of the freshly 

 infected water with sterile gelatine or agar, and then poured the 

 liquid transparent mass on a shallow glass dish, and allowed it to 

 set or stiffen there into a transparent film, considerable differences 

 were observable according as such a plate-culture was left exposed to 

 the light or covered up from it. 



In the latter case each of the numerous invisible spores contained 

 in the drop of water, and now evenly distributed through the trans- 

 parent mass of solidified gelatine, &c, germinated out in a few hours, 

 and formed a colony of bacteria so dense and opaque that the 

 previously clear gelatine, &c, became studded closely with them, and 

 looked as if peppered over with a dense cloud of dust ; whereas in 

 the former case the light was found to have killed so many of the 

 spores that only a very few colonies were formed on the plate at 

 wide intervals, and separated from one another by the portions of 

 transparent gelatine in which no growth had occurred. 



[Plate-cultures shown illustrating this.] 



It must be borne in mind that the gelatine, which remains trans- 

 parent in this case, is just as richly charged with spores as in the 

 other case, only the spores remain invisible owing to their minute- 

 ness ; they are dead, and therefore cannot germinate and grow and 

 develope into the colonies which render the gelatine, &c, opaque. 



It was pretty obvious that if the light was the agent which thus 

 killed the exposed spores, the effect ought to be better brought out 

 by exposing part only of the gelatine plate, and covering up the rest. 

 This I did as follows, by a modification of a method long known to 

 botanists and practised by myself for years. A large quantity of 

 spores was thoroughly shaken up in distilled sterile water, so that 

 every cubic centimetre of the water contained from 1,000,000 to 

 5,000,000 of the invisible spores. Then about 5 drops of this 

 water was mixed with gelatine, warmed until quite liquid, and the 



