276 Professor Ward [April 27, 



damage, though they do not survive prolonged boiling. Here is 

 sufficient evidence, however, to prove that we are not dealing with 

 spores which are at all tender to high temperatures, but with forms 

 which are wholly unscathed by any such temperatures as they have to 

 endure on exposure to the sunlight. 



Yet an exposure of half-an-hour or so to the direct rays of the sun 

 kills these spores outright. 



But direct sunlight is not necessary, as the following experiment 

 shows. Four culture cells were prepared as described, each hanging 

 drop having a small number (about 15 to 25) of spores in it, and 

 placed each with a thermometer control cell as follows. One was 

 under a darkened bell jar ; a second under a large flat-bottomed glass 

 dish filled with clear water ; a third under a similar dish filled with 

 deep-blue ammoniacal solution of cupric oxide ; and the fourth 

 under a similar dish of deep orange-coloured solution of potassium 

 bichromate. [Demonstration of methods of exposure.] The first, 

 therefore, was excluded altogether from the light, while the second 

 received all the light passing through water and clear glass. The 

 third was exposed to the feeble light passing through the very 

 deep-coloured copper solution, and from which all red and yellow 

 rays were cut off; while the fourth was exposed to the red and 

 yellow rays only, all the blue-violet being excluded by the screen. 



Now mark the conditions of exposure, as recorded on the chart. 

 They were all outside, at the north, and in the shade of a building, 

 so that no direct sunlight reached them at all — only the lights from 

 the blue sky and clouds ; moreover the temperature was low, and so 

 nearly the same in each culture cell as that of the surrounding air, 

 that even the most sceptical critic would not ascribe the results to the 

 minute differences, even if we did not know from the other evidence 

 that he would be wrong in doing so. 



These cultures were exposed for five hours as described, and then 

 all brought into the laboratory and placed in the dark at the same 

 temperature — 21° C. — a temperature chosen as being the most favour- 

 able for their further development. After a sufficient interval of 

 time, the lengths of all the germinated bacilli in the drops were 

 measured, and the means and averages taken, and plotted on sectional 

 paper, with the results now shown. 



[Curves of average growth of these thrown on screen.] 



The results were that the spores which had been excluded from 

 the light and those which had been sheltered behind the bichromate 

 from all blue-violet rays, germinated out normally, and the bacilli 

 rapidly developed and grew ; those exposed to the blue rays (though 

 of feeble intensity) were so far retarded, that the curve of growth is 

 markedly depressed ; while those exposed to the more intense light 

 coming through the clear water, and therefore to more intense light 

 containing more of the blue and violet, were killed altogether, for 

 they refused to germinate in all cases and even after several clays' 

 nursing. 



