Professor Tyndall on Germs. 



thus reduced to ocular demonstration. Let us inquire a little 

 more closely into the character of the particles which produce the 

 life. Pour Eau-de-Cologne into water, a white precipitate renders 

 the liquid milky. Or, imitating Brucke, dissolve clean gum mastic 

 in alcohol, and drop it into water, the mastic is precipitated and 

 milkiness produced. If the solution be very strong the mastic 

 separates in curds; but by gradually diluting the alcoholic solu- 

 tion we finally reach a point where the milkiness disappears, the 

 liquid assuming, by reflected light, a bright cerulean hue. It is, 

 in point of fact, the color of the sky, and is due to a similar cause, 

 namely, the scattering of light by particles, small in comparison 

 to the size of the waves of light. 



When this liquid is examined by the highest microscopic power 

 it seems as uniform as distilled water. The mastic particles, 

 though innumerable, entirely elude the microscope. At right 

 angles to a luminous beam passing among the particles they dis- 

 charge perfectly polarised light. The optical deportment of the 

 floating matter of the air proves it to be composed, in part, of 

 particles of this excessively minute character. When the track of 

 a parallel beam in dusty air is looked at horizontally through a 

 Nicol's prism, in a direction perpendicular to the beam, the longer 

 diagonal of the prism being vertical, a considerable portion of the 

 light from the finer matter is extinguished. The coarser motes, 

 on the other hand, flash out with greater force, because of the 

 increased darkness of the space around them. It is among the 

 finest ultra-microscopic particles that the author shows the matter 

 potential as regards the development of Bacterial life is to be 

 sought. 



But though they are beyond the reach of the microscope, the 

 r these particles, foreign to the atmosphere but floating 

 in It, IS as certain as if they could be felt between the fingep, or 

 seen by the naked eye. Supposing them to augment in magnitude 

 until they come, not only within range of the microscope, but 

 within range of the unaided senses. Let it be assumed that our 

 knowledge of them under these circumstances remains as defective 

 as it is now — that we do not know whether they are germs, parti- 

 cles of dead organic dust, or particles of mineral matter. Suppose 

 a vessel (say a flower-pot) to be at hand filled with nutritious 

 earth, with which we mix our unknown particles ; and that m 

 forty-eight hours subsequently buds and V)l:i(les of \vell-<letiiied 

 cresses and grasses appear above the >*«>il. Supix'-i- tli'' e\}'iii- 

 ment when repeated over and over airain to yield the ^.iiuf un- 

 varying result. What would be our comcIumod y >liotil.l we 

 regard those living plants as the products of de:t<l^ du-t or iiiini-r;il 

 particles; or should we regard them as the ofispring <>1 ItMiig 

 seeds ? The reply is unavoidable. We should undoubtedly coq- 



