304 FRAGMENTS OF SCIENCE. 



cos tails, it inflates the cellular tissue of the whole body. Yet this occa- 

 sions no alarm to the surgeon (although if the blood in the pleura were to 

 putrefy, it would infallibly occasion dangerous suppurative pleurisy). 

 Why air introduced into the pleural cavity through a wounded lung 

 should have such wholly different effects from that entering directly 

 through a wound in the chest was to me a complete mystery until I heard 

 of the germ-theory of putrefaction, when it at once occurred to me that 

 it was only natural, that air should be filtered of germs by the air-pas- 

 sages, one of whose offices is to arrest inhaled particles of dust, and pre- 

 vent them from entering the air-cells. 



I shall have occasion to refer to this remarkable hy- 

 pothesis further on. 



The advocates of the germ-theory, both of putrefaction 

 and epidemic disease, hold that both arise, not from the air, 

 but from something contained in the air. They hold, more- 

 over, that " something " to be not a vapor nor a foreign 

 gas, nor indeed a molecule of any kind, but a particle? 

 The term " particulate " has been used in the Reports of 

 the Medical Department of the Privy Council to describe 

 this supposed constitution of contagious matter ; a,nd 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 importance, because in the treat- 

 ment of particles methods are available which it would be 

 futile to apply to molecules. 



Application of iMminous Beams to researches of this 

 nature. 



My own interference with this great question, while 

 sanctioned by eminent names, has been also an object of 



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. 



