DUST AND DISEASE. 27 



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 paHicle.^ The term ' particulate ' has been used 

 by Mr. Simon in the Eeports of the Medical Department 

 of the Privy Council to describe this supposed constitu- 

 tion of contagious matter ; and Dr. Sanderson's experi- 

 ments 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 importance, because in the treatment of par- 

 ticles methods are available ■which it would be futile to 

 apply to molecules. 



The Lumvnous Beam as a means of Besearch. 



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 



' 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 draw is this: ihe a^om 

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

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

 aggregation of atoms or molecules. 



