SPONTANEOUS GENERATION. 301 



annihilated. But the action of living contagia extends 

 beyond the domain of the surgeon. The power of re- 

 production and indefinite self-multiplication which is 

 characteristic of living things, coupled with the un- 

 deviating fact of contagia 'breeding true/ has given 

 strength and consistency to a belief long entertained by 

 penetrating minds, that epidemic diseases generally are 

 the concomitants of parasitic life. ' There begins to be 

 faintly visible to us a vast and destructive laboratory of 

 nature wherein the diseases which are most fatal to ani- 

 mal life, and the changes to which dead organic matter 

 is passively liable, appear bound together by what must 

 at least be called a very close analogy of causation/ * 

 According to this view, which, as I have said, is daily 

 gaining converts, a contagious disease may be defined 

 as a conflict between the person smitten by it and a 

 specific organism which multiplies at his expense, ap- 

 propriating his air and moisture, disintegrating his tis- 

 sues, or poisoning him by the decompositions incident 

 to its growth. 



During the ten years extending from 1859 to 1869, 

 researches on radiant heat in its relations to the gas- 

 eous form of matter occupied my continual attention. 

 When air was experimented on, I had to cleanse it 

 effectually of floating matter, and while doing so I was 

 surprised to notice that, at the ordinary rate of transfer, 

 such matters passed freely through alkalis, acids, alco- 

 hols, and ethers. The eye being kept sensitive by dark- 

 ness, a concentrated beam of light was found to be a 

 most searching test for suspended matter both in water 

 and in air a test indeed indefinitely more searching 

 and severe than that furnished by the most powerful 

 microscope. With the aid of such a beam I examined 



* Report of the Medical Officer of the Privy Council, 1874, p. 5. 



