effected in the constituents of the air by the wool, by proving that the cotton-wool might be dispensed with altogether, and perfectly free access left between the exterior air and that in the experimental flask.  If the neck of the flask is drawn out into a tube and bent downwards; and if, after the contained fluid has been carefully boiled, the tube is heated sufficiently to destroy any germs which may be present in the air which enters as the fluid cools, the apparatus may be left to itself for any time and no life will appear in the fluid.  The reason is plain.  Although there is free communication between the atmosphere laden with germs and the germless air in the flask, contact between the two takes place only in the tube; and as the germs cannot fall upwards, and there are no currents, they never reach the interior of the flask.  But if the tube be broken short off where it proceeds from the flask, and free access be thus given to germs falling vertically out of the air, the fluid, which has remained clear and desert for months, becomes, in a few days, turbid and full of life.

[Footnote 9:  Lectures to Working Men on the Causes of the Phenomena of Organic Nature, 1863. (See Vol.  II. of these Essays.)]

These experiments have been repeated over and over again by independent observers with entire success; and there is one very simple mode of seeing the facts for one’s self, which I may as well describe.

Prepare a solution (much used by M. Pasteur, and often called “Pasteur’s solution”) composed of water with tartrate of ammonia, sugar, and yeast-ash dissolved therein.[10] Divide it into three portions in as many flasks; boil all three for a quarter of an hour; and, while the steam is passing out, stop the neck of one with a large plug of cotton-wool, so that this also may be thoroughly steamed.  Now set the flasks aside to cool, and, when their contents are cold, add to one of the open ones a drop of filtered infusion of hay which has stood for twenty-four hours, and is consequently hill of the active and excessively minute organisms known as Bacteria.  In a couple of days of ordinary warm weather the contents of this flask will be milky from the enormous multiplication of Bacteria.  The other flask, open and exposed to the air, will, sooner or later, become milky with Bacteria, and patches of mould may appear in it; while the liquid in the flask, the neck of which is plugged with cotton-wool, will remain clear for an indefinite time.  I have sought in vain for any explanation of these facts, except the obvious one, that the air contains germs competent to give rise to Bacteria, such as those with which the first solution has been knowingly and purposely inoculated, and to the mould-Fungi.  And I have not yet been able to meet with any advocate of Abiogenesis who seriously maintains that the atoms of sugar, tartrate of ammonia, yeast-ash, and water, under no influence but that of free access of air and the ordinary temperature, re-arrange themselves and give rise to the protoplasm of Bacterium.  But the alternative is to admit that these Bacteria arise from germs in the air; and if they are thus propagated, the burden of proof that other like forms are generated in a different manner, must rest with the assertor of that proposition.