At last one man, a shrewd, intelligent, and influential observer, a man of genius, gave to the whole question a new phasis. Since his day his hypothesis (for we shall presently find that as yet it deserves no better name) has undergone a variety of modifications, as was to be expected, in no way, however, affecting the practical deductions originally drawn from it by its author. A brief history of this curious episode in medicine, honoured by some with the pompous title of “a revolution in sanitary science,” will fitly precede the inquiry on which I am about to enter. Like the small white cloud warning the navigator of the approaching tornado, this hypothesis, from its first appearance as a humble essay in a monthly journal, has repeatedly assumed, by force of circumstances, gigantic dimensions. Of it, as of Rumour, it may be truly said, _Vires acquirit eundo_: it gathers strength from motion. As is usual in England, a machinery has been tacked to it of a character most heterogeneous, but withal so heavy as already to threaten to surpass endurance--of the truth of which remark no further evidence need be adduced than the modest demand of six millions sterling to depurate or cleanse the Thames of those very materials which, as a first experiment, and by no means an unprofitable one, the Sanitary Board ordered and compelled the inhabitants of London to throw into it. A brief history of this remarkable phasis of sanitary science, as it is called, may prove acceptable to my readers. CHAPTER II. THEORIES OF MACCULLOCH. About thirty years ago, as I have already remarked, one of the most distinguished practical geologists of this or any other country directed his attention to a subject of much greater difficulty than the classification of rocks, and their subdivision into primary, secondary, volcanic, and transition. His object was to discover the origin or cause of those fatal diseases which, under the names of fever, dysentery, plague, rheumatism, &c., render the position of man on the globe so precarious, his life at times so brief, valueless to himself or to others, his prospects so gloomy; in brief, by tracing to its origin, if possible, the active agent of such woes to man, to destroy its fatal influence by practical hygienic measures. In a word, Dr. Macculloch hoped, by discovering the cause, to devise the means either of effectually destroying malaria--using the term, however, in a sense at that time peculiar to himself--or so to mitigate its effects as to render it less destructive to mankind.