20 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



In vegetable physiology some of the most striking 

 researches have been on the effect produced by rays of 

 light of different refrangibility. Daubeny, Draper, and 

 Sachs have shown that the light of the red end of the 

 spectrum is more effective than that of the blue, so far 

 as the decomposition of carbon dioxide (carbonic acid) 

 is concerned. 



Nothing could have appeared less likely than that 

 researches into the theory of spontaneous generation 

 should have led to practical improvements in medical 

 science. Yet such has been the case. Only a few 

 years ago Bacteria seemed mere scientific curiosities. 

 It had long been known that an infusion say, of hay 

 would, if exposed to the atmosphere, le found, after a 

 certain time, to teem with living forms. Even those 

 few who still believe that life would be spontaneously 

 generated in such an infusion, will admit that these 

 minute organisms are, if not entirely, yet mainly, de- 

 rived from germs floating in our atmosphere ; and if 

 precautions are taken to exclude such germs, as in the 

 careful experiments especially of Pasteur, Tyndall, and 

 Roberts, everyone will grant that in ninety-nine cases 

 out of a hundred no such development of life will take 

 place. In 1836-7 Cagniard de la Tour and Schwann 

 independently showed that fermentation was no mere 

 chemical process, but was due to the presence of a 

 microscopic plant. But, more than this, it has been 

 gradually established that putrefaction is also the work 

 of microscopic organisms. Thirty years, however, 

 elapsed before these important discoveries received any 

 practical application. 



At length, however, these facts have led to most 



