284 BIOLOGY AND ITS MAKERS 
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 show- 
ing 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 experi- 
ments 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 ex- 
periments were published in 1775, and were generally re- 
garded 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 Spal- 
lanzani’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 
