A CENTURY OF CHEMISTRY. II7 



end of the scale of being we may note the attraction 

 of some micro-organisms to light. ThusEngelmann's 

 Bacterium photometricum — rod-like purple microbes 

 — not only crowd in a drop of water under the mi- 

 croscope to the particular spot on which the smallest 

 possible beam of light is focussed, but when a micro- 

 scopic spectrum is projected on the field " selects " 

 the area whose colour is that which is most absorbed 

 by their minute bodies. 



One other illustration of the chemical action of 

 light upon living creatures may be given, namely, the 

 destructive effect of light upon many kinds of mi- 

 crobes, both in the air and in culture-solutions. We 

 are accustomed to think of light as life-giving, but it 

 also kills. And the fact is significant and full of 

 practical suggestion that sunlight is the most potent, 

 universal, and economical antagonist of some of our 

 worst enemies. How exactly the light kills the bac- 

 teria remains somewhat uncertain, but it is com- 

 monly believed that it induces too rapid oxidation, 

 that it makes the minute organisms live so fast that 

 they die. 



Photochemical research has been as yet in great 

 part concerned with different modes of measuring 

 the chemical activity of light. One of the most suc- 

 cessful methods takes advantage of the fact that light 

 induces a mixture of equal volumes of chlorine and 

 hydrogen to form hydrogen chloride (Draper, 1843 ; 

 Bunsen and Eoscoe, 1857). This led to the estab- 

 lishment of the conclusions that the chemical action 

 is proportional to the light intensity, that equal 

 chemical effects are produced when the products of 

 light intensity and time of exposure are equal, that 

 substances are affected differently by different rays, 

 and so on. How it is that light induces chemical 



