class similarly situated, in all respects except in the exposure to that one cause.[5] The effects of a cause of depression on a class are sometimes visible in the greater fatality of common accidents. An excess of mortality to a class is almost always found, on examination, to be traceable to an adequate cause. From the external circumstances of a class of the population, a confident expectation may be formed of the sum of vitality of the class, though nothing could be separately predicated of a single individual of it. If the former vulgar notions were correct as to the salubrity of the stenches which prevail in towns, the separate as well as the combined results of these several supposed causes of salubrity must be to expel fevers and epidemics from the most crowded manufacturing districts, and to advance the general health of the inhabitants above that of the poorer rural population; but all such fallacies are dissipated by the dreadful facts on the face of the mortuary records showing a frequency of deaths, and a reduction of the mean duration of life, in proportion to the constancy and the intensity of the combined operation of these same causes.[6] § 6. The observations of the effects of such emanations on the general health of classes of human beings have been corroborated by experiments on animals. § 7. Another doctrine more extensively entertained than that above noticed, is, that although putrid emanations are productive of injury, they are not productive of specific disease, such as typhus. The medical witnesses say, that they were exposed to such emanations in dissecting-rooms, where bodies of persons who have died of small-pox, typhus, scarlatina, and every species of disease, are brought; that they pursued their studies in such places, and were unaware of typhus or other disease having been taken by the students in them, though that disease was frequently caught by students whilst attending the living in the fever wards.[7]