28 
PROFESSOR TYNDALL ON THE OPTICAL DEPORTMENT OF THE 
This conclusion was strengthened by an experiment easily made and of high signi- 
ficance in relation to this question. It had been pointed out by Professor Lister, of 
Edinburgh *, that air which has passed through the lungs is known to have lost its 
power of causing putrefaction. Such air may mix freely with the blood without risk of 
mischief; and that truly great scientific Surgeon had the penetration to ascribe this immu- 
nity from danger to the filtering power of the lungs. Prior to my becoming acquainted 
with this hypothesis in 1869, 1 had demonstrated its accuracy in the following manner f. 
Condensing in a dark room, and in dusty air, a powerful beam of light, and breathing 
through a glass tube (the tube actually employed was a lamp-glass, rendered warm in a 
flame to prevent precipitation) across the focus, a diminution of the scattered light 
was first observed. But towards the end of Fig. 1. 
the expiration the white track of the beam 
was broken by a perfectly black gap, the 
blackness being due to the total absence from 
the expired air of any matter competent to 
scatter light. The experimental arrangement 
is represented in fig. 1, where g represents 
the heated lamp-glass, and b the gap cut out 
of the beam issuing from the lamp L. The 
deeper portions of the lungs were thus proved 
to be filled with optically pure air, which, as 
such, had no power to generate the organisms essential to the process of putrefaction J. 
It seemed that this simple method of examination could not fail to be of use to workers 
in this field. They had hitherto proceeded less by sight than by insight, being in 
general unable to see the physical character of the medium in which their experi- 
ments were conducted. But the method has not been much turned to account ; and 
this year I thought it worth while to devote some time myself to the more complete 
demonstration of its utility. 
I also wished to free my mind, and if possible the minds of others, from the uncer- 
tainty and confusion which now beset the doctrine of “spontaneous generation.” 
Pasteur has pronounced it “ a chimera,” and expressed the undoubting conviction that 
this being so it is possible to remove parasitic diseases from the earth. To the medical 
profession, therefore, and through them to humanity at large, this question, if the 
* Introductory Lecture before tbe University. t Proc. Roy. Inst. vol. vi. p. 9. 
+ “ No putrefaction,” says Cora, “ can occur in a nitrogenous substance if it be kept free from the entrance 
of new Bacteria after those which it may contain have been destroyed. Putrefaction begins as soon as Bacteria, 
even in the smallest numbers, are accidentally or purposely introduced. It progresses in direct proportion to 
tbe multiplication of the Bacteria ; it is retarded when the Bacteria (for example, by a low temperature) develop 
a small amount of vitality, and is brought to an end by all influences which either stop the development of the 
Bacteria, or kill them. All bactericidal media are therefore antiseptic and disinfecting.” — Beitrdge zur Biologie 
der PJlanzen, zweites Heft, 1872, p. 203. 
Fig. 1. 
