"And it is further certain that these living particles are so minute that the assumption of their suspension in ordinary air presents not the slightest difficulty. On the contrary, considering their lightness and the wide diffusion of the organisms which produce them, it is impossible to conceive that they should not be suspended in the atmosphere in myriads." The experimental means by which these facts are proved may be briefly stated:-- I. The air contains solid particles. Professor Tyndall has shown, as all who have read "Dust and Disease" know to their own discomfort, that the purest common air, when submitted to a beam of electric light, renders the track of that beam visible. Ergo, it must contain solid particles capable of scattering light. II. These particles are mostly destructible by heat, or may be mechanically strained off. He has shown this by the fact that common air which has passed through a red-hot tube, or through a filter of cotton-wool, will no longer render the track of the electric beam visible. III. Many of these particles are germs. Schulze and Schwann proved that when air is passed through red-hot tubes, then through a fluid which is capable of affording a nidus to the germs, if present, no development of life takes place. Similarly Schr[oe]der established the same fact by using a strainer of cotton-wool. Further, Pasteur gave an additional proof by microscopical examination, as well as by a direct experiment. He passed air through gun-cotton, dissolved this in ether; and in the collodion germs were clearly recognizable. Also he plunged a piece of cotton-wool through which air had been strained into an experimental fluid. This fluid soon swarmed with forms of life. IV. The experimental fluid may be inoculated by simple exposure to air as well as by any fluids known to contain living forms; _e.g._, if the fluid be placed in an open vessel, living forms soon make their appearance. Yet supposing the aperture of the vessel, instead of pointing vertically upwards, be turned obliquely or downwards, the fluid will remain clear for an indefinite time. Similarly a drop of an infusion containing living forms added to the experimental fluid soon causes it to swarm with life. The forms that appear are the same in either case. V. The experimental fluid cannot give rise itself to these forms. It is known as Pasteur's solution, and consists of water, ammonium, tartrate, sugar, and yeast ash. Hence there is no organic matter in it. If proper care be taken, it may be kept for an indefinite time. VI. The germs are so minute that in many cases, even when known to be present, they are scarcely visible to the highest microscopic powers. They must be universally diffused, as any organic infusion left exposed soon swarms with the forms to which they give rise.