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F we look back over the field of chemistry, we find that we can easily discern well-characterized periods in its development. At first, in this subject, as in all others, came the period of chaos, during which relations of similar facts were not recognized nor suspected. No defined object was in view; and the development during this period was due almost entirely to accidental observations of facts which presented themselves to men in pursuing their ordinary occupations. Gradually we find that a certain class of men began to make use of chemical facts, as far as they were then known, for a very definite purpose. This was, to convert ordinary base metals into that metal which possessed the greatest value—gold. This purpose gave a powerful incentive to the study of chemical phenomena, and, under the influence of the natural passion which affected a comparatively large number of men, the subject of chemistry grew apace. But the impossibility of accomplishing the great problem of the alchemists became more and more apparent. No gold was made from baser metals, and no genuine philosopher's stone was discovered; no panacea for all diseases was revealed. A reaction in scientific opinion then began, which led to very much modified views concerning the purpose of chemistry, until about the time of Paracelsus, who was both physician and chemist, we find that the opinion prevailed very generally, among those who were most active in investigating chemical phenomena, that the changes which take place in the animal body, under normal conditions, are nothing but chemical changes; that a disturbance of these normal changes causes the different varieties of disease; and, finally, that the treatment of disease must consist in administering such chemical substances as would restore the normal conditions. Paracelsus started these ideas, and others developed them, until they took the exaggerated form comprised in the above statements. According to these ideas, medicine was considered as a branch of chemistry, very much as metallurgy is now considered as a branch of chemistry. Hence the physicians of the date of which I am speaking—i. e., from the early part of the sixteenth until some time in the seventeenth century—regarded chemistry as the one important subject for those who were to deal with disease. Without a knowledge of this subject they could not comprehend the processes of life; without it they could not understand disease; without it they could not intelligently administer remedies.