62 THE FOUNDATIONS OF SCIENCE 



further idea that something from the outside had 

 come in and attacked the crop, which was expressed 

 in the view that either the moon, the Milky Way, 

 or some heavenly body had let fall an infecting sub- 

 stance. The remedy proposed, namely that a young 

 dog should be sacrificed, probably to conciliate the 

 constellation of Sirius (the dog-star), is however a 

 part of the classical treatment of the subject which 

 would find no analogue in modern science. 



The putrefaction of decaying animal and vege- 

 table matter, also, had been a subject of speculation 

 from the earliest times, and the fermentation pro- 

 duced by yeast has also been known throughout 

 history. Yet it was not till about 1838 that Schwann 

 (1810-1882) discovered that the activity of yeast was 

 due to the growth and multiplication of living cells, 

 and that putrefaction was caused by a similar process. 



These results were extended about 1855 by Pasteur 

 (1822-1895), who traced all such changes to the in- 

 troduction of living cells or germs of cells, and thus 

 destroyed the old idea of spontaneous generation. 

 Pasteur detected the specific microbes of chicken 

 cholera and the silk-worm disease, and gradually 

 our knowledge of microbic diseases has been ex- 

 tended, chiefly by inoculation experiments on living 

 animals, and we have learnt how to guard against 

 some of the worst of these microscopic but deadly 

 enemies. To this knowledge we can trace most of 

 the modern improvements in sanitation and hygiene. 



