nic
molecules,' and the latter assuming the existence of a special
'vegetative force' which drew the molecules together so as to form
living things. On the other side, we have the celebrated Abbe Lazzaro
Spallanzani, who in 1777 published results counter to those announced
by Needham in 1748, and obtained by methods so precise as to
completely overthrow the convictions based upon the labours of his
predecessor. Charging his flasks with organic infusions, he sealed
their necks with the blowpipe, subjected them in this condition to the
heat of boiling water, and subsequently exposed them to temperatures
favourable to the development of life. The infusions continued
unchanged for months, and when the flasks were subsequently opened no
trace of life was found.

Here I may forestall matters so far as to say that the success of
Spallanzani's experiments depended wholly on the locality in which he
worked. The air around him must have been free from the more obdurate
infusorial germs, for otherwise the process he followed would, as was
long afterwards proved by Wyman, have infallibly yielded life. But
his refutation of the doctrine of spontaneous generation is not the
less valid on this account. Nor is it in any way upset by the fact,
that others in repeating his experiments obtained life where he
obtained none. Rather is the refutation strengthened by such
differences. Given two experimenters equally skilful and equally
careful, operating in different places on the same infusion, in the
same way, and assuming the one to obtain life while the other fails to
obtain it; then its well-established absence in the one case proves
that some ingredient foreign to the infusion must be its cause in the
other.

Spallanzani's sealed flasks contained but small quantities of air, and
as oxygen was afterwards shown to be generally essential to life, it
was thought that the absence of life observed by Spallanzani might
have been due to the lack of this vitalising gas. To dissipate this
doubt, Schulze in 1836 half filled a flask with distilled water to
which animal and vegetable matters were added. First boiling his
infusion to destroy whatever life it might contain, Schulze sucked
daily into his flask air which had passed through a series of bulbs
containing concentrated sulphuric acid, where all germs of life
suspended in the air were supposed to be destroyed. From May to
August this process was continued without any development