288 THE FliOATING-MATTEB OF THK AIR. 



annihilated. But the action of living contagia extend? 

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

 production and indefinite selt-multiplication which is 

 characteristic of living things, coupled with the 

 undeviating 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 miUtiplies at his expense, 

 appropriating his air and moisture, disintegrating his 

 tissues, 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 matter passed freely through alkalis, acids, alcohols, 

 and ethers. The eye being kept sensitive by darkness, 

 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 

 ' Eeport of the Medical Officer of the Privy Council, 1874, p. 6. 



