264 Professor Ward [April 27, 



whole rapidly poured in an even thin film over the flat floor of one of 

 these thin clear glass dishes. In five or ten minutes the gelatine had 

 set to a thin transparent film, containing say 5,000,000 invisible 

 spores. The bottom of the dish, outside, was now covered with a zinc 

 stencil-plate, through which a letter of the alphabet (say E) was cut, 

 and every other part of the dish wrapped over with tin-foil and black 

 paper, so that no light could reach the film except that which passed 

 through the E-shaped opening in the metal stencil-plate in contact 

 with the thin glass on which the gelatine film rests. 



[Demonstration of method of making and exposing such plates.] 



The plate, letter downwards, was then suspended over a plane 

 silvered glass mirror, placed at an angle such that the rays of the 

 sun could be reflected up vertically through the letter, for two, three 

 or more hours. 



After this process of exposure, the coverings and stencil-plate 

 were removed from the plate, and the latter put in a warm dark 

 incubator. The film showed no effects, but was still an even trans- 

 parent sheet of gelatine, because the spores, dead or alive, are so 

 minute that they in no way perceptibly interfere with the clearness 

 of the gelatine, and none have had time as yet to germinate out. 



On allowing such an exposed plate to remain, in the dark, at a 

 temperature suitable for promoting the germination of the living 

 spores, however, a marked and striking result is obtained in from 

 twenty-four to forty-eight hours or so ; for it is found that those spores 

 which lie imbedded in the parts of the gelatine sheltered from the 

 light, by the foil and other coverings, have germinated out normally 

 and developed into visible colonies, which render the gelatine opaque, 

 and which are so numerous that they run into one another, and so 

 produce a general grey, white or other coloured layer, according to 

 the kind of bacterium, &c, used, whereas those spores in the E-shaped 

 area exposed to light do not alter, because they are dead — killed by 

 the light — and therefore do not affect the clearness and transparency 

 of the gelatine, being invisible on account of their minuteness. 



Consequently we see a transparent letter E picked out from an 

 opaque matrix of growing bacteria. 



[Photographs and plates shown.] 



Buchner, by a method essentially similar to this, showed that a 

 few hours' exposure to the intense light of the sun in summer suffices 

 for this, but I showed that the much feebler rays of the winter sun 

 are capable, even after reflection from a mirror, of totally killing 

 the spores of the anthrax bacillus, at a temperature so low that even 

 the gelatine ordinarily employed in cultures, and which runs at 

 29° C, is not melted. Since the spores employed will withstand very 

 much higher temperatures — 55° C. to 60° C. — for many hours, it is 

 clear that these experiments are conclusive against the supposition 

 that the bactericidal action is due to the temperature. Having 

 established this point for this and a number of other forms, 

 including some mould fungi and yeasts, the next step was to settle 



