which diseases of similar type or character affect the population. It will be sufficient to mention its first great class—that of zymotic diseases. This term zymotic is derived from a Greek word meaning ferment, and has reference to a change analogous to that of fermentation occurring in the blood by the infinite multiplication of disease-germs. Such affections chiefly comprise fevers par excellence—the epidemic, endemic, and contagious or infectious disorders—which suddenly attack masses of people, which spring from different sorts of malaria, or from specific communicable poisons; contaminate the atmosphere and water, and decimate in a brief time civil and military communities. We read in sacred history of whole armies having been suddenly swept away, as that of Sennacherib, which, while besieging Jerusalem, lost 185,000 men in a single night under the deadly breath of the destroying angel—a beautiful metaphor, probably, for the swift and invisible blow of the pestilence. It has been well remarked that these diseases distinguish one country from another, one year from another. They have formed epochs in chronology, and, as Niebuhr has shown, "have influenced not only the fall of cities, such as Athens and Florence, but of empires."
This great class of maladies is the index of salubrity; it is this class which varies to the greatest extent in different climates and seasons, which modifies the fatality of other kinds of disease, and which constitutes the principal difference between the health of different peoples and periods.
A general and uniform system of death-registration among nations renders easy what would otherwise be impracticable, viz., constant international exchanges and comparisons, not simply confined to individual affections, but applicable as well to immense groups of cognate diseases. In this manner statistics of mortality assume vast importance, and present for our consideration manifold questions of a physical, social, and political character. They determine the laws which regulate the duration of life; they indicate in what manner those laws have been or are being infringed, and afford bases for calculations materially affecting the interests of mankind. Statistics are far from being the barren array of figures ingeniously and laboriously combined into columns and tables, which some persons are apt to consider them. They constitute rather the ledger of the people, in which, as the merchant in his books, the citizen can read at once all the results of a week, a month, a year, or series of years, and can deduce the profit or the loss which has accrued to the account of vitality, morals, education, wealth, power. And it has been well said that "science has nothing to offer more inviting in speculation than the laws of vitality, the variations of those laws in the two sexes at different ages, and the influence of civilization, occupation, locality, seasons, and other physical agencies, either in generating diseases or in improving the public health."