66 Royal Society: — Prof. J. Tyndall on the 



temperature of Erom 00° to 7o° Fahr., all fell into putrefaction in 

 the course of from two to four clays. No matter where the infu- 

 sions were placed, they were infallibly smitten in the end. The 

 number of the tubes containing infusions was multiplied till it 

 reached six hundred, but not one of them escaped infection. 



In no single instance, on the other hand, did the air which had 

 been proved moteless by the searching beam prove itself, even 

 when raised to temperatures varying from 80° to 90°, to possess 

 the least power of producing Bacterial life or the associated 

 phenomena of putrefaction. The power of developing such life 

 in atmospheric air, and the power of scattering of light, are 

 thus proved to be indissolubly united. 



The sole condition necessary to cause these long-dormant infu- 

 sions to swarm with active life is the access of the floating matter 

 of the air. After having remained for four months as pellucid 

 as distilled water, the opening of the back door of the protecting 

 case, and the consequent admission of the mote-laden air, sufficed 

 in three days to render the infusions putrid and full of life. 



That such life arises from mechanically suspended particles is 

 thus reduced to ocular demonstration. 



Let us inquire a little more closely into the character of the par- 

 ticles which produce the life. Pour eau de Cologne into water, a 

 white precipitate renders the liquid milky. Or, imitating Briicke, 

 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 dilu- 

 ting the alcoholic solution we finally reach a point where the milki- 

 ness disappears, the liquid assuming, by reflected light, a bright 

 cerulean hue. It is, in point of fact, the colour 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 discharge per- 

 fectly polarized light. The optical deportment of the floating matter 

 of the air proves it to be composed in part of particles of this ex- 

 cessively minute character. When the track of a parallel beam in 

 dusty air is looked at horizontally through a Mcol'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 dark- 

 ness of the space around them. It is among the finest ultra-micro- 

 scopic particles that the author shows that the matter potential as 

 regards the development of Bacterial life is to be sought. 



But though they are bej^ond the reach of the microscope, the ex- 

 istence of these particles, foreign to the atmosphere but floating 

 in it, is as certain as if they could be felt between the fingers or 

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



