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posed that there is a logical difficulty in the way of proving that 
life does not grow out of dead matter — that to attempt to prove 
this is to attempt to prove a negative. But every man of science 
believes that the quantity of matter is constant, and that the 
quantity of energy is constant, although these propositions equally 
involve the negatives, that matter and energy never appear or dis- 
appear, but merely undergo transformations. But although there 
is no absurdity, there is a great difficulty in the way of proving 
that living beings are always produced from pre-existing living 
beings. It is difficult to make our experiments under precisely 
the conditions under which nature works, and at the same time to 
exclude the possibility of the presence of living beings. If we 
boil our liquid in order to kill its living contents, it may be said 
that we change its chemical character, and deprive it of the power 
of producing life ; if we shut it up in a hermetically closed vessel, 
we prevent that access of air which may be essential to the produc- 
tion of life from lifeless matter. Mr Lister has shown us how we 
may obtain milk, urine, and blood quite free from living beings, and 
keep them liquid for any length of time freely exposed to air with- 
out any risk of the entrance of living things, and he has shown us 
that if this be done no living things ever appear in the liquid. In 
his experiments we see two samples of the same substance treated, 
with one exception, in exactly the same way; in the one sample 
life is abundantly developed, in the other not at all. Can any 
reasonable man doubt that this striking difference of result is due 
to the one only difference of treatment; and this difference of 
treatment is merely that in the one case living things have had 
access to the substances, in the other they have been excluded? 
In all other respects the two samples have been exposed to pre- 
cisely the same influences. With all respect for those experi- 
menters who, not having taken Mr Lister’s precautions, have 
arrived at different results, I express my conviction that it has 
been definitely proved that life is continuous, that living matter 
cannot be produced by a chemical process, and that every living 
thing is descended from some previously existing living parent. 
IV. Another aspect of this paper is of less general scientific 
interest, but of much greater practical importance. Mr Lister’s 
investigation forms the scientific basis of the system of antiseptic 
surgery, with which his name will always be associated. The 
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