by penetrating minds, that epidemic diseases
generally are the concomitants of parasitic life. 'There begins to be
faintly visible to us a vast and destructive laboratory of nature
wherein the diseases which are most fatal to animal life, and the
changes to which dead organic matter passively liable, appear bound
together by what must least be called a very close analogy of
causation.' [Footnote: Report of the Medical Officer of the Privy
Council, 1874, p. 5.] According to this view, which, as I have said,
is daily gaining converts, a contagious disease may be defined a
conflict between the person smitten by it and a specific organism
which multiplies at his expense, appropriating his air and moisture,
disintegrating his tissues, or poisoning him by the decompositions
incident to its growth.

*****

During the ten years extending from 1859 to 1869, researches on
radiant heat in its relations to the gaseous form of matter occupied
my continual attention. When air was experimented on, I had to
cleanse it effectually of floating matter, and while doing so I was
surprised to notice that, at the ordinary rate of transfer, such
matter passed freely through alkalis, acids, alcohols, and ethers. The
eye being kept sensitive by darkness, a concentrated beam of light was
found to be a most searching test for suspended matter both in water
and in air--a test indeed indefinitely more searching and severe than
that furnished by the most powerful microscope. With the aid of such
a beam I examined air filtered by cotton-wool; air long kept free from
agitation, so as to allow the floating matter to subside; calcined
air, and air filtered by the deeper cells of the human lungs. In all
cases the correspondence between my experiments and those of
Schroeder, Pasteur, and Lister in regard to spontaneous generation was
perfect. The air which they found inoperative was proved by the
luminous beam to be optically pure and therefore germless. Having
worked at the subject both by experiment and reflection, on Friday
evening, January 21, 1870, I brought it before the members of the
Royal Institution. Two or three months subsequently, for sufficient
practical reasons, I ventured to direct public attention to the
subject in a letter to the Times. Such was my first contact with this
important question.

This letter, I believe, gave occasion for the first public utterance
of Dr. Bastian in relation to this subject. He did me the honour