272 HARRY MARSHALL WARD 



Ward showed that subsidence could not be entirely relied on, as 

 the sediment might easily become the source of re-infection. 

 The effect of sunlight required more critical examination. 



It was known that the spores of anthrax were liable to be 

 washed into rivers. Ward determined to study this as the most 

 extreme type of pathogenic infection. As it is undoubtedly the 

 most deadly micro-organism known, and Ward proposed to deal 

 with it on a large scale, it implied no small degree of courage. 

 He found that the spores of anthrax were effectually killed by 

 a few hours' exposure to even the reflected light of a low winter 

 sun. It was clear that this was due to the direct action of the 

 light and not to any heating effect, apart from the fact that they 

 will tolerate boiling for a few minutes. It was further shown 

 that there was no foundation for the theory of Roux and 

 Duclaux that their death was due to poisoning by products of 

 oxidation of the food-medium. Proof of this, indeed, was hardly 

 required, for Pasteur had shown that the bacteria floating in 

 the atmosphere are mostly dead. Were it not so, no surgical 

 operation would be possible. To the bactericidal effect of sun- 

 light is equally to be attributed the absence of bacteria from the 

 High Alps. 



The next point was to ascertain to what rays the effect was 

 due. The spores of anthrax are so minute that, when mixed in 

 large numbers with gelatine, they do not affect its transparency. 

 A plate of glass coated with the mixture is at first clear, but 

 ceases to be so if kept in the dark, owing to the germination of 

 the spores. Ward found, in fact, that a photograph could be 

 printed with it, the darkening being the reverse of that of a 

 silver plate. After experiments with coloured screens he com- 

 pletely solved the problem in 1893, with the aid of apparatus 

 supplied by Sir Oliver Lodge and some advice from Sir Gabriel 

 Stokes, by photographing the spectrum on such a plate. It was 

 at once seen that the destructive effect was due to rays of high 

 refrangibility, and, what was extremely important, extended 

 to, and found its maximum in, the ultra-violet. The same 

 results were obtained with the typhoid bacillus. He made the 

 suggestion that the arc light might be used for the disinfection 

 of hospitals and railway carriages. 



