PUTEEFACTION AND INFECTION. 75 



them. But much more than this may be afiSrmed. 

 The electric or the solar beam is a far more powerful 

 and searching test in this matter than the microscope. 

 In the foregoing pages I have more than once described 

 the clearness of my protected infusions, after months 

 of exposure, as equal to that of distilled water. So far 

 is this from being an exaggeration, that it falls short of 

 the truth ; for I have never seen distilled water so free 

 from suspended particles as the protected infusions 

 prove themselves to be. When for months a transparent 

 liquid thus defies the scrutiny of the searching beam, 

 maintaining itself free from every speck which could 

 scatter light as a Bacterium scatters it — when, moreover, 

 an adjacent infusion, prepared in precisely the same 

 way, but exposed to the ordinary air, becomes first 

 hazy, then turbid, and ends by wholly shattering the 

 concentrated beam into irregularly scattered light, I 

 think we are entitled to conclude that Bacteria are as- 

 certainly absent from the one as they are present in the 

 other. (See Note I. at the end of this paper.) 



For the right interpretation of scientific evidence 

 something more than mere sharpness of observation is 

 requisite, very keen sight being perfectly compatible 

 with very weak insight. I was therefore careful to have 

 my infusions inspected by biologists, not only trained 

 in the niceties of the microscope, but versed in all the 

 processes of scientific reasoning. Their conclusion is 

 that it would simply weaken the demonstrative force of 

 the experiments to appeal to the microscope at all. 



§ 16. Suspended Particles in Air and Water; 

 their relation to Bacteria. 



Examined by the concentrated solar rays, or by the 

 condensed electric beam, the floating matter of the air 



