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466 beyond all reasonable limits this Introductory Note. On the old Baconian lines of observation and experiment the work is carried on. The intercommunication of scientific thought plays here a most important part. It will probably have been noticed that, while physiologists and physicians in England and elsewhere were drawing copiously from the store of facts furnished by the researches of Pasteur, that admirable investigator long kept himself clear of physiology and medicine. There is, indeed, reason to believe that he was spurred on to his most recent achievements by the papers of Burdon Sanderson, Koch, and others. The union of scientific minds is, or ought to be, organic. They are parts of the same body, in which every member, under penalty of atrophy and decay, must discharge its due share of the duty imposed upon the whole. Of this "body," a short time since, England provided one of the healthiest limbs; but round that limb legislation has lately thrown a ligature, which threatens to damage its circulation and to divert its energies into foreign channels. In observational medicine one fine piece of work may be here referred to—the masterly inquiry of Dr. Thorne Thorne into the outbreak of typhoid fever at Caterham and Redhill. Hundreds were smitten by this epidemic, and many died. The qualities of mind illustrated in Dr. Thome's inquiry match those displayed by William Budd in his memorable investigation of a similar outbreak in Devonshire. Dr. Budd's process was centrifugal—tracing from a single case, in the village of North Tawton, the ravages of the fever far and wide. Dr. Thome's process was centripetal—tracing the epidemic backward, from the multitude of cases first presented, to the single individual whose infected excreta, poured into the well at Caterham, were the cause of all.

The essays here presented to the reader belong to the A B C of the great subject touched upon in the foregoing Note. The two principal ones, namely, Essays II and III, were prepared for the Royal Society, and are published in the "Philosophical Transactions" for 1876 and 1877. But, though written for that learned body, I sought to render their style and logic so clear as to render them accessible to any fairly cultivated mind. The essays on "Fermentation" and "Spontaneous Generation" have already appeared elsewhere; while the first essay, on "Dust and Disease," has been for some years before the public. It may be regarded as a kind of popular introduction to the more strenuous and original labors which follow it.

The essay most likely to try the reader's patience is Number III. On the whole, however, and particularly in its bearings on the germ theory of disease, it is probably the most important of all. The difficulties which sometimes beset the experimenter in these investigations are best illustrated by this essay. It shows, to my mind in a very impressive manner, the analogy of the spread of infection among organic infusions with its mode of propagation among human beings.