into the rivers, to poison, in a certain sense, the waters on which man lives, rendering their banks, if not pestilential, at least most unpleasant as human abodes; canals are dug, vast reservoirs are formed, which in time give rise by mismanagement to fevers, intermittent and others; the minerals of the earth are quarried and placed on the soil, mines are dug, and from them waters are discharged into the neighbouring streams, strongly poisoned with the metallic ores. To imagine that an influence thus affecting earth, air, and water can proceed and increase without affecting human life, can be overcome by habit, does not require to be met by counter-influences originating in the experience and reasoning of man himself, is a supposition which the history of large cities refutes. The influence is reciprocal. When man thus acts on the three elements of nature by which he lives, they react on him, and it is this reaction he is called on to meet and to overcome as best he can. It is a question of reason and experiment--that is, of science and of simple observation; simple observation and experiment taught the native Peruvians the value of guano, for science had at that time no standing on the American continent; and now the chemist steps in and explains why it was that the experiment proved successful. Whether his explanation be satisfactory or not, touches not the question; though proved to be erroneous in a single instance, as it possibly is in regard of this very Peruvian guano, science stands on too secure a basis to require any defence from me.