244 THE ORIGIN AND PROPAGATION OF DISEASE. 



betray their presence, under such circumstances, by a luminous track 

 l)assing lengthwise through the tube. In following out these investi- 

 gations, he arrived at two rather unexpected and somewhat important 

 results. 



The first was that the solid corpuscles in suspension in the atmo- 

 sphere are to a very great extent of organic origin. As the dust which 

 settles upon our shelves and moldings, when collected and examined, 

 was known to be largely composed of mineral particles, it was naturally 

 thought that the floating impurities of the air were probably of a similar 

 character. But when Professor Tyndall allowed the illuminated electric 

 beam to pass directly over the flame of a spirit-lami3, he found that a per- 

 fectly dark space was cut out of the beam — wreaths of darkness rising up- 

 ward from the flame and taking the place, at that point, of the luminous 

 particles. A hydrogen flame, a red-hot poker, or a bundle of incandescent 

 platinum wires, placed in a similar position, produced the same effect. 

 That is, the floating molecules of the air, which have the power of dis- 

 persing the light of the electric beam, instead of being rendered incan- 

 descent and more luminous than before by a high temperature, are de- 

 stroyed by it and resolved into transparent vapors. They are therefore 

 combustible and organic. 



This fact is of great importance, because it gives us our first definite 

 knowledge with regard to the nature of the greater part of the atmo- 

 spheric dust. If this be really the channel by which germs of disease 

 are disseminated in the form of distinct corpuscles, we shall never be 

 fully satisfied until we have been able to examine and recognize them, 

 so as to place the fact bej^ond a doubt. But thus far it has been found 

 exceedingly difficult to capture an-d place under the microscope the mi- 

 nute ingredients of the atmospheric imparities. This has, it is true, 

 been accomplished for a very few of the simpler and well-known fungus 

 germs ; but as for the larger portion of the floating particles which a 

 sunbeam brings into view, our knowledge has heretofore been limited 

 to the mere fact of their existence. It is therefore a great step to learn 

 that, whatever may be their remaining characters, they are at all events 

 of organic origin. The minute size of these bodies, and especially their, 

 lightness, no doubt explain fully the persistence with which they are 

 raised and disseminated by the gentlest air-currents, while the heavier 

 particles of a mineral nature are more readily deposited in the form of 

 dust upon all inclined and horizontal surfaces. 



The second fact brought out by the observations of Professor Tyndall 

 is the extreme difficulty of excluding from the air all of its finer dust- 

 particles, so as to obtain it absolutely free from suspended matter. The 

 electric beam, passing through the glass tube before spoken of, was used 

 by the experimenter as a test for the presence of solid impurities in the 

 air contained in the tube. As the beam is, of course, invisible by itself, 

 and becomes perceptible to the eye only by meeting with bodies capable 

 of reflecting its light, any luminosity in its track through the tube would 



