Biology and Its Makers / With Portraits and Other Illustrations

by William A. Locy

Fig. 90.—Lazzaro Spallanzani, 1729-1799.

Spallanzani took nutrient fluids, such as the juices of vegetables and meats which had been extracted by boiling, placed them in clear flasks, the necks of which were hermetically sealed in flame, and afterward immersed them in boiling water for three-quarters of an hour, in order to destroy all germs that might be contained in them. The organic infusions of Spallanzani remained free from change. It was then, as now, a well-known fact that organic fluids, when exposed to air, quickly decompose and acquire a bad smell; they soon become turbid, and in a little time a scum is formed upon their surface. The fluids in the flasks of Spallanzani remained of the same appearance and consistency as when they were first introduced into the vessel, and the obvious conclusion was drawn that microscopic life is not spontaneously formed within nutrient fluids.

"But Needham was not satisfied with these results, and with a show of reason maintained that such a prolonged boiling would destroy not only germs, but the germinative, or, as he called it, the 'vegetative force' of the infusion itself. Spallanzani easily disposed of this objection by showing that when the infusions were again exposed to the air, no matter how severe or prolonged the boiling to which they had been subjected, the infusoria reappeared. His experiments were made in great numbers, with different infusions, and were conducted with the utmost care and precision" (Dunster). It must be confessed, however, that the success of his experiments was owing largely to the purity of the air in which he worked, the more resistant atmospheric germs were not present: as Wyman showed, long afterward, that germs may retain their vitality after being subjected for several hours to the temperature of boiling water.

Schulze and Schwann.—The results of Spallanzani's experiments were published in 1775, and were generally regarded by the naturalists of that period as answering in the negative the question of the spontaneous generation of life. Doubts began to arise as to the conclusive nature of Spallanzani's experiments, on account of the discovery of the part which oxygen plays in reference to life. The discovery of oxygen, one of the greatest scientific events of the eighteenth century, was made by Priestley in 1774. It was soon shown that oxygen is necessary to all forms of life, and the question was raised: Had not the boiling of the closed flasks changed the oxygen so that through the heating process it had lost its life-giving properties? This doubt grew until a reëxamination of the question of spontaneous generation became necessary under conditions in which the nutrient fluids were made accessible to the outside air.

In 1836 Franz Schulze, and, in the following year, Theodor Schwann, devised experiments to test the question on this new basis. Schwann is known to us as the founder of the cell-theory, but we must not confuse Schulze with Max Schultze, who established the protoplasm doctrine. In the experiments of Schulze, a flask was arranged containing nutrient fluids, with a large cork perforated and closely fitted with bent glass tubes connected on one side with a series of bulbs in which were placed sulphuric acid and other chemical substances. An aspirator was attached to the other end of this system, and air from the outside was sucked into the flask, passing on its way through the bulbs containing the chemical substances. The purpose of this was to remove the floating germs that exist in the air, while the air itself was shown, through other experiments by Schwann, to remain unchanged.

Tyndall says in reference to these experiments: "Here again the success of Schulze was due to his working in comparatively pure air, but even in such air his experiment is a risky one. Germs will pass unwetted and unscathed through sulphuric acid unless the most special care is taken to detain them. I have repeatedly failed, by repeating Schulze's experiments, to obtain his results. Others have failed likewise. The air passes in bubbles through the bulbs, and to render the method secure, the passage of the air must be so slow as to cause the whole of its floating matter, even to the very core of each bubble, to touch the surrounding fluid. But if this precaution be observed water will be found quite as effectual as sulphuric acid."

Schwann's apparatus was similar in construction, except that the bent tube on one side was surrounded by a jacket of metal and was subjected to a very high temperature while the air was being drawn through it, the effect being to kill any floating germs that might exist in the air. Great care was taken by both experimenters to have their flasks and fluids thoroughly sterilized, and the results of their experiments were to show that the nutrient fluids remained uncontaminated.

These experiments proved that there is something in the atmosphere which, unless it be removed or rendered inactive, produces life within nutrient fluids, but whether this something is solid, fluid, or gaseous did not appear from the experiments. It remained for Helmholtz to show, as he did in 1843, that this something will not pass through a moist animal membrane, and is therefore a solid. The results so far reached satisfied the minds of scientific men, and the question of the spontaneous origin of life was regarded as having been finally set at rest.

III. The Third Period. Pouchet.—We come now to consider the third historical phase of this question. Although it had apparently been set at rest, the question was unexpectedly opened again in 1859 by the Frenchman Pouchet, the director of the Natural History Museum of Rouen. The frame of mind which Pouchet brought to his experimental investigations was fatal to unbiased conclusions: "When, by meditation," he says, in the opening paragraph of his book on Heterogenesis, "it was evident to me that spontaneous generation was one of the means employed by nature for the production of living beings, I applied myself to discover by what means one could place these phenomena in evidence." Although he experimented, his case was prejudiced by metaphysical considerations. He repeated the experiments of previous observers with opposite results, and therefore he declared his belief in the falsity of the conclusions of Spallanzani, Schulze, and Schwann.