Louis Pasteur: His Life and Labours

by René Vallery-Radot

The same experiments may be repeated with slightly alkaline liquids, such as milk, the precaution being taken of raising them to a temperature higher than that of 100 degrees Centigrade.

The great interest of Pasteur's method consists in its proving unanswerably that the origin of life, in infusions which have been heated to the boiling point, is solely due to the solid particles suspended in the air. Of gas, electricity, magnetism, ozone, things known or unknown, there is nothing in ordinary atmospheric air which, apart from these solid particles, can cause the fermentation or putrefaction of the infusions.

Lastly, to convince the most prejudiced minds, and to leave no contradiction standing, Pasteur showed one of these bulbs with the sinuous neck which he had prepared and preserved for months and years. The bulb was covered with dust. 'Let us,' said he, 'take up a little of this outside dust on a bit of glass, porcelain, or platinum, and introduce it into the liquid; the following day you will find that the infusion, which up to this time remained perfectly clear, has become turbid, and that it behaves in the same manner as other infusions in contact with ordinary air.'

If the bulb be tilted so as to cause a little drop of the clear infusion to reach the extremity of the bent part of the neck where the dust particles are arrested, and if this drop be then allowed to trickle back into the infusion, the result is the same—turbidity supervenes and the microscopic organisms are developed. Finally, if one of those bulbs which have stood the test of months and years without alteration be several times shaken violently, so that the external air shall rush into it, and if it be then placed once more in the stove, life will soon appear in it.

In 1860 the Academy of Sciences had offered a prize, the conditions of which were stated in the following terms:

'To endeavour by well-contrived experiments to throw new light upon the question of spontaneous generation.' The Academy added that it demanded precise and rigorous experiments equally well studied on all sides; such experiments, in short, as should render it possible to deduce from them results free from all confusion due to the experiments themselves. Pasteur carried away the prize, and no one, it will be acknowledged, deserved it better than he. Nevertheless, to his eyes, the subject was still beset with difficulties. In the hot discussions to which the question of spontaneous generation gave rise, the partisans of the doctrine continually brought forward an objection based on an opinion already referred to, and first enunciated by Gay-Lussac. As already known to the reader, Gay-Lussac had arrived at the conclusion that, in Appert's process, one condition of the preservation of animal and vegetable substances consisted in the exclusion of oxygen.

Even this proposition was soon improved upon, and it became a current opinion in science that the smallest bubble of oxygen or of air which might come in contact with a preserve would be sufficient to start its decomposition. The partisans of spontaneous generation—the heterogenists—thenceforward threw their objections to Pasteur into this form:

'How can the germs of microscopic organisms be so numerous that even the smallest bubble of air contains germs capable of developing themselves in every organic infusion? If such were the case the air would be encumbered with organic germs.' M. Pouchet said and wrote that they would form a thick fog, as dense as iron.

But Pasteur showed that the interpretation of Gay-Lussac's experiment, with respect to the possible alteration of preserves by a small quantity of oxygen gas, was quite erroneous. If, after a certain time, an Appert preserve contains no oxygen, this is simply because the oxygen has been gradually absorbed by the substances of the preserve, which are always more or less chemically oxidisable. But in reality it is easy to find oxygen in these preserves. Pasteur did not fail to perceive that the interpretation given to Gay-Lussac's experiment was wrong in another particular. He proved the fallacy of the assumption that the smallest quantity of air was always capable of producing microscopic organisms.

More thickly spread in towns than in the country, the germs become fewer in proportion as they recede from human habitations. Mountains have fewer than plains, and at a certain height they are very rare.