ON DUST AND DISEASE. 157 



be filtered of germs by the air -passages, one of whose offices 

 is to arrest inhaled particles of dust, and prevent them from 

 entering the air-cells. 



I shall have occasion to refer to this remarkable hypo- 

 thesis farther on. 



The advocates of the germ theory, both of putrefac- 

 tion and epidemic disease, hold that both arise, not 

 from the air, but from something contained in the air. 

 They hold, moreover, that this 'something' is not a 

 vapour nor a gas, nor indeed a molecule of any kind, 

 but a particle. 1 The term ' particulate ' has been used 

 in the Keports of the Medical Department of the Privy 

 Council to describe this supposed constitution of con- 

 tagious matter ; and Dr. Sanderson's experiments render 

 it in the highest degree probable, if they do not actually 

 demonstrate, that the virus of small-pox is ' particulate.' 

 Definite knowledge upon this point is of exceeding im- 

 portance, because in the treatment of particles methods/ 

 are available which it would be futile to apply to molt 

 cules. 



The Luminous Beam as a means of Research. 



My own interference with this great question, while 

 sanctioned by eminent names, has been also an object 

 of varied and ingenious attack. On this point I will 

 only say that when angry feeling escapes from behind 

 the intellect, where it may be useful as an urging force, 

 and places itself athwart the intellect, it is liable to 



1 As regards size, there is probably no sharp line of division 

 between molecules and particles ; the one gradually shades into 

 the other. But the distinction that I would draw is this: the atom 

 or the molecule, if free, is always part of a gas, the particle is never 

 so. A particle is a bit of liquid or solid matter, formed by the 

 aggregation of atoms or molecules. 



