278 Professor Ward [April 27, 



gets down as far as 400 meters, and Forel got effects with photographic 

 paper at 45 to 100 meters in the Lake of Geneva, and so on, depths 

 quite sufficient for my purposes, though I hope that more accurate 

 and extended information will yet be obtained on the subject. 



Some consequences of this view of the destruction of bacteria in 

 water by light are of the utmost importance. For instance, we can 

 understand how the typhoid bacillus, if it falls into turbid dirty 

 water in the summer, may multiply at the high temperature and at the 

 expense of the food-materials dissolved from the organic matters in 

 suspension, and this at its pleasure, so to speak, because the sus- 

 pended particles of matter, to which the turbidity is due, favour it in 

 (1) supplying it with food, (2) absorbing heat and helping to raise 

 the temperature of the water, and (3) impeding the penetration of the 

 destructive light-rays. 



The results of the investigation also suggest explanations for 

 many other facts which have hitherto been unexplained. Thus 

 Pasteur and Miquel pointed out some time ago that the germs 

 floating about in the air are for the most part dead ! This we can 

 explain by the bactericidal action of the sun's rays, just as we can 

 also probably explain the freedom from germs of the Alps. 



Again, Martinaud has shown that certain yeasts which normally 

 vegetate on the exterior of ripening grapes, are destroyed if the 

 sunshine is very intense ; and Giunti has stated that the ingress of 

 sunlight hinders acetic fermentation, a process which depends on 

 the life-processes of a well-known bacterium. 



I have myself observed many cases where the free access to 

 light is inimical to the germination and development of fungi of 

 various kinds, and Elfving has recently studied very thoroughly 

 the action of light on a common mould-fungus with astonishing 

 results, quite intelligible in the light of the foregoing investigation. 

 These matters are of more significance in connection with the spread 

 of the potato and other diseases in dull warm weather than I have 

 time to explain here ; and probably have a wide bearing on the 

 deterioration of forest and agricultural soils exposed to the sun, as 

 well as on many questions of greenhouse practice too numerous to 

 even mention. 



They also bear on the question of sun-baths, and the whole 

 hygiene of sunshine treatment ; as well as sun-burn, " tanning," 

 snow-blindness, &c. But still more startling probabilities must be 

 looked at, though I have brought my lecture now to a point where I 

 must remember that your patience in listening to me should not be 

 abused. 



We have seen that in my experiments certain colour-screens are 

 efficacious in so cutting off the access of the destructive rays, that the 

 protected germs, bacilli, fungus-spores, &c, can germinate and develop 

 as easily as if no light at all was playing on them. It struck me 

 during the progress of these experiments that just such, colour-screens 

 are very common in Nature, in cases where just such spores and tender 



