permanent results are at last arrived at. Modern science arose from the exhaustion of previous methods of thought. The earlier philosophy speculated concerning Nature, and sought after her truths in the depths of the human mind. All that high genius and varied intellectual power could do was done, but to no purpose until the searchers for truth changed their attitude to Nature, and began to inquire of her by the simple and despised methods of experimental investigation. Dr. Priestley, first of all men, approached the problem of the constitution of the air in this spirit, and was even compelled to devise the contrivances by which gaseous bodies could be manipulated. He was on the right track, he had struck the true method, and magnificently did Nature reward his sagacity and his wisdom. Of course, for the Greeks or the Romans, or the schoolmen of the middle ages, to have discovered oxygen, would have been impossible. Only with the decline of their modes of thought could new methods arise, and only through the apprenticeship of generations in the field of physical investigation were men prepared to pass to the subtler search of the inner nature of material things. The discovery of oxygen, therefore, came in its due time in the mental unfolding of humanity; and while to Dr. Priestley undoubtedly belongs the honor of having first disclosed and identified it, others would quickly have plucked the ripened fruit if he had not; and in point of fact oxygen was independently discovered shortly after by the Swedish chemist Scheele, who also discovered chlorine in 1774.
But, if the discovery of oxygen formed a great epoch in our advancing knowledge of the constitution of Nature, its influence was no less profound upon the advance of chemical science. We are accustomed to regard chemistry as a kind of knowledge that is peculiarly modern, but it is really very old, and has had a long course of development. Liebig has stated that the completion of a science implies three stages or operations. There are 1.—The ascertainment of the properties of things by observation and experiment; 2. The bringing of them into relation by principles or ideas; and, 3. The application of mathematics, or subjecting the phenomena to the test of quantitative investigation. In chemistry, the first of these stages runs back to antiquity. The ancients knew many facts and made many empirical experiments in the arts which were of a chemical nature. They knew seven metals—gold, silver, mercury, copper, iron, tin, and lead. They also knew various preparations of zinc, antimony, and arsenic, and must have had a very considerable knowledge of metallurgical processes. They had also a knowledge of glass, pottery, soap, dyes, pigments, precious stones, asphalt, alum, starch, beer, and many other substances which, if not exact, was still so positive as to guide them in the processes of manufacture. This kind of knowledge of the properties of bodies must have gradually increased, and when we come down to the time of Gheber, the Arabian, who wrote a thousand years ago, we find that this species of information had not only greatly increased, but had become more definitely chemical in character, while laboratory operations were systematically practised. Gheber, for example, knew the properties of common salt, potash, soda, saltpetre, ammonia, copperas, borax, corrosive sublimate, oxide of copper, metallic arsenic, compounds of sulphur with the metals, and the methods of preparing sulphuric and nitric acids, aqua-regia, litharge, and the operations of distillation, sublimation, smelting, and a great number of chemical processes, as they were practised down to the end of the eighteenth century. By the alchemists these facts were immensely multiplied, forming a vast body of knowledge concerning the chemical proper-
