-violet rays to pass than does glass. The red-yellow-green to the left
of F were without effect. (H. M. W.) ]

No milk is free from bacteria, because the external orifices of the
milk-ducts always contain them, but the forms present in the normal fluid
are principally those which induce such changes as the souring or "turning"
so frequently observed in standing milk (these were examined by Lord Lister
as long ago as 1873-1877, though several other species are now known), and
those which bring about the various changes and fermentations in butter and
cheese made from it. The presence of foreign germs, which may gain the
upper hand and totally destroy the flavours of butter and cheese, has led
to the search for those particular forms to which the approved properties
are due. A definite bacillus to which the peculiarly fine flavour of
certain butters is due, is said to be largely employed in pure cultures in
American dairies, and in Denmark certain butters are said to keep fresh
much longer owing to the use of pure cultures and the treatment employed to
suppress the forms which cause rancidity. Quite distinct is the search for
the germs which cause undesirable changes, or "diseases"; and great strides
have been made in discovering the bacteria concerned in rendering milk
"ropy," butter "oily" and "rancid," &c. Cheese in its numerous forms
contains myriads of bacteria, and some of these are now known to be
concerned in the various processes of ripening and other changes affecting
the product, and although little is known as to the exact part played by
any species, practical applications of the discoveries of the decade
1890-1900 have been made, _e.g._ Edam cheese. The Japanese have cheeses
resulting from the bacterial fermentation of boiled Soja beans.

[v.03 p.0168]

[Sidenote: Thermophilous bacteria.]

That bacterial fermentations are accompanied by the evolution of heat is an
old experience; but the discovery that the "spontaneous" combustion of
sterilized cotton-waste does not occur simply if moist and freely exposed
to oxygen, but results when the washings of fresh waste are added, has led
to clearer proof that the heating of hay-stacks, hops, tobacco and other
vegetable products is due to the vital activity of bacteria and fungi, and
is physiologically a consequence of respiratory processes like those in
malting. It seems fairly established that when the preliminary heating
process of fermentation is drawing to a cl