RELATION OF LIVING ORGANISMS TO PUTREFACTION.
Now two questions arise—questions which have been already alluded to in the historical sketch given on pp. 493, 494. They are these : What is the origin of these bacteria? and what is their relation to putrefaction, from which they are never absent? To each question there are two answers: to the first, as to the origin, the old answer is expressed by the phrase': "spontaneous generation ;" that is to say, organic substances may give birth to them under certain conditions of heat and moisture; the modern answer is: They have always parents like to them. To the second question, their relation to putrefaction, one answer is: They merely accompany it; the, other is: They cause it, and without them it cannot be. Now, in spite of experiments like those of Schroeder and Dusch, men of high scientific attainments held and yet hold the view of spontaneous gen eration. It does seem absurd to believe that one cannot expose anything to the atmosphere kr a single instant without there being de posited on it invisible particles of dust, in cluding seeds, spores, or germs of bacteria or bacteria themselves, which await only a favour able opportunity to develop and attack, if it is liable to attack, the substance to which they have adhered. But this apparent absurdity, by a series of beautiful experiments, was shown by Prof. Tyndall to be a fact. Everyone knows that if a room be darkened by closing the shut ters, and if a beam of sunlight be allowed to enter by a chink or crack in the shutter, its track will be revealed by myriads of dancing motes that catch the light and disperse it. The track of a beam of the lime or electric light will be shown in the same way. But for these (lancing particles of dust the light would only light up the object on which it fell, its pathway would be invisible. Tyndall constructed a chamber whose top, floor, back, and sides were of wood, and whose front was of glass. In the floor were openings through which teat-tubes were passed, their mouths opening into the chamber. A strong beam of light was thrown
into the chamber, its path being clearly visible owing to the dancing particles. The chamber was then allowed to stand undisturbed; and three days later, when a beam of light was again thrown into it, its pathway was invisible. This showed that the air was perfectly free from suspended particles, which had all attached themselves to the sides or fallen to the bottom of the chamber. Into the test-tubes an organic fluid, capable, therefore, of decomposition, was carefully poured through an opening in the roof. The projecting ends of the tubes were allowed to dip into a bath of brine, which was boiled for 5 minutes. The bath was then re moved and the chamber allowed to remain. For months it stood, the fluid showing no signs whatever of putrefactive change, while the fluid in similar tubes, subject to the same treatment, but standing in the open air of the laboratory, rapidly decomposed. But a few days after the chamber had been disturbed, so as to raise the dust, the fluid in every tube gave way and was found to swarm with bacteria. Experi ments with all sorts of infusions gave precisely similar results. Moreover, Tyndall showed that air strained through cotton-wool, or* heated by passing through a. red-hot tube, no longer re vealed the pathway of the beam of light—was deprived, that is, of solid particles, thus ex plaining fully how air admitted to flasks plugged with wool did not produce decay in any organic material in the flasks. In truth these and mul titudes of other experiments show convincingly that the air is everywhere laden with solid particles, including germs which, sown on suit able soil, rapidly multiply, their multiplication being accompanied by all the stages of putre faction. They exist everywhere, but more thickly spread in towns than in the country, becoming fewer as one recedes from human habitations; but they are present, nevertheless, in the atmosphere far removed from human dwellings, on the heights of the Alps as in the most densely-peopled valleys.