310 A FURTHER CONTRIBUTION TO THE 
to that of Cohn,! based upon absolute morphological characters, is entirely 
untrustworthy. In order to determine the species of any particular specimen 
it is necessary to take into account not merely its appearance, but also the 
character of the medium in which it occurs. Even then mere morphology will 
often entirely fail us unless we are able to ascertain the physiological characters. 
And even these appear by no means constant ; for we shall in the present paper 
see reason to believe that one and the same bacterium may differ at different 
times in its fermentative effects on one and the same organic solution. 
It is obvious that to trace the modifications of any one such organism 
through a series of successive habitats would be an utter impossibility if bacteria 
or any kind of fungi were liable to be evolved from the mere chemical con- 
stituents of the liquids employed; and thus the investigation, though not 
undertaken for the purpose of combating the doctrine of spontaneous generation, 
has afforded the strongest possible evidence against it, and in favour of the 
germ theory of fermentative changes. For even in organic liquids such as 
milk, in which spontaneous generation has been said to be most liable to occur, 
it required only a rigorous attention to experimental details to ensure the com- 
plete absence of either organic development or fermentative change, except 
where organisms were intentionally introduced. But when this was done, the 
particular species used for inoculation grew unmixed with others, attended by 
the chemical alterations characteristic of it. 
In order to enable the reader to give credence to my statements, it is essential 
that I should describe in detail the mode of procedure in its most improved 
form. Let us take, as an example, the case of boiled milk. The first thing 
to be done is to ensure that the interior of the vessel in which the liquid is to 
be heated shall be free from any living organisms. This is done by subjecting 
a Florence flask to a very high temperature, after providing that the air which 
enters on cooling shall be effectually filtered of living dust by passing through 
asbestos which, I find, answers this purpose quite satisfactorily. The asbestos 
is placed, in a mass about a quarter of an inch thick, between two layers of tin- 
foil sufficiently broad and long for wrapping round the junction of the neck 
of the flask and a glass cap that covers its mouth ; and when it has been so 
arranged, fine iron wire is tied tightly round, so as to compress it firmly as well 
as retain it in position after the outer layer of tinfoil has been dissipated by 
* It is, however, only just to Professor Cohn to state that he dwells largely upon the different : 
physiological effects of different supposed species of bacteria, and sometimes makes them a ground 
of classification, more especially in the group of ‘ pigment bacteria ’, which he distinguishes from others 
on account of the remarkable colouring matters to which they give rise. Nevertheless he relies in the 
main on absolute morphological characters. See ‘ Untersuchungen iiber Bacterien ’, von Dr. Ferdinand 
Cohn. Beitrdge zur Biologie der Pflanzen, Zweites Heft, Breslau, 1872. 
