THE ORIGIN AND PROPAGATION OF DISEASE. 245 



indicate the existence of floating particles in the contained air. It was 

 found by this test that, contrary to expectation, air which had been 

 passed in succession through tubes containing fragments of glass wetted 

 with concentrated sulphuric acid and with a solution of caustic potash 

 immediately before its admission to the tube still contained similar impu- 

 rities. Even when it had been allowed to bubble through the liquid sul- 

 phuric acid and caustic potash, the electric beam still revealed the pres- 

 ence of suspended matter. Either the contact of these liquids could 

 not be made sufficiently complete with all parts of the air passing 

 through them, or for some other reason they failed to purify it entirely 

 of its organic ingredients. 



But a burning temperature succeeded in accomplishing the object. 

 A platinum tube, containing a roll of platinum gauze, was placed in a 

 small gas-fnrnace, and heated to incandescence. The air was slowly 

 drawn through this apparatus, and then, when admitted into the glass 

 experimental tube, was found to be absolutely free from suspended ma- 

 terial. The electric beam j)assed through it, from end to end, without 

 exhibiting any luminosity 5 the interior of the tube being perfectly dark, 

 while outside of it the track of the beam was lighted up by floating dust- 

 particles, as before. A tube filled with air in this condition was said by 

 Professor Tyndall to be optically empty ; that is, it contained only the 

 transparent gases of the atmosphere without any admixture of corpuscles 

 capable of reflecting light. 



The air may also be freed from its suspended impurities by filtration 

 through cotton-wool. A quantity of this substance was packed rather 

 tightly in a glass tube, so as to form a thick, porous plug. The air, 

 drawn through this tube into the experimental receiver, showed itself to 

 be non-luminous, and consequently pure. This property of cotton-wool 

 has been often utilized in experiments on the necessary conditions of 

 organic development, and it is found that air thus thoroughly filtered 

 does not carry ^ith it any germs capable of producing infusorial or veg- 

 etative life. 



Finally, Professor Tyndall also demonstrated that air, which has once 

 passed through the deeper portion of the respiratory passages, has un- 

 dergone a similar filtration. If the expired breath be made to traverse 

 a luminous electric beam, it at first produces upon it no visible effect. 

 But toward the end of the expiration it makes itself evident by a dimi- 

 luitiou in the amount of dispersed light, and at last extinguishes com- 

 pletely the luminosity of the beam, cutting out at the point of contact 

 a perfectly dark space. Precisely by what mechanism this filtration is 

 accomplished it is not easy to say, since the smallest of the pulmonary 

 passages in the human lung have a diameter of not less than one-fifth of 

 a millimeter; but there can be little doubt, in i)oint of fact, that by the 

 time the air reaches the ultimate divisions of the respiratory cavities, it 

 is thoroughly freed from the ordinary suspended matter which it con- 

 tained on entering the trachea. 



