t application of the action of bactericidal rays to the
cure of lupus is, however, an extension of the same discovery. Even when
the light is not sufficiently intense, or the exposure is too short to kill
the spores, the experiments show that attenuation of virulence [v.03
p.0169] may result, a point of extreme importance in connexion with the
lighting and ventilation of dwellings, the purification of rivers and
streams, and the general diminution of epidemics in nature.

[Sidenote: Bacteria and cold.]

As we have seen, thermophilous bacteria can grow at high temperatures, and
it has long been known that some forms develop on ice. The somewhat
different question of the resistance of ripe spores or cells to extremes of
heat and cold has received attention. Ravenel, Macfadyen and Rowland have
shown that several bacilli will bear exposure for seven days to the
temperature of liquid air (-192deg C. to -183deg C.) and again grow when
put into normal conditions. More recent experiments have shown that even
ten hours' exposure to the temperature of liquid hydrogen -252deg C. (21deg
on the absolute scale) failed to kill them. It is probable that all these
cases of resistance of seeds, spores, &c., are to be connected with the
fact that completely dry albumin does not lose its coagulability on heating
to 110deg C. for some hours, since it is well known that completely ripe
spores and dry heat are the conditions of extreme experiments.

[Sidenote: Pathogenic bacteria.]

No sharp line can be drawn between pathogenic and non-pathogenic
Schizomycetes, and some of the most marked steps in the progress of our
modern knowledge of these organisms depend on the discovery that their
pathogenicity or virulence can be modified--diminished or increased--by
definite treatment, and, in the natural course of epidemics, by alterations
in the environment. Similarly we are unable to divide Schizomycetes sharply
into parasites and saprophytes, since it is well proved that a number of
species--facultative parasites--can become one or the other according to
circumstances. These facts, and the further knowledge that many bacteria
never observed as parasites, or as pathogenic forms, produce toxins or
poisons as the result of their decompositions and fermentations of organic
substances, have led to important results in the applications of
bacteriology to medicine.

[Illustration: FIG. 20.--The ginger-beer plant.

A. One of the brain-like gelatinous