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 Schrœder 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.
Such an array of facts, proved experimentally over and over again, must convince the most tenacious sceptic, and he may feel inclined to agree with the opinion expressed in the following passage from Sir B. Brodie:[142]—
"Crites. Then, if I understand you rightly, you have arrived at these conclusions. First, that there was a time when this earth was not in a fit state for the maintenance of either animal or vegetable life. Secondly, that in its present condition there is no evidence of any law being in operation which would account for any living beings being called into existence except as the offspring of other living beings which previously existed; and that from these premises we cannot fail to arrive at this further conclusion, that the first introduction of life on earth must have been by some special act of the creative power, of which we have no experience at present.
"Eubulus. I suspect that this, really and truly, is all we actually know on the subject."