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THE POPULAR SCIENCE MONTHLY.

are possible only when facts have been observed, and definite knowledge has been acquired in special directions. In the sixteenth century, alchemy, having failed to discover the philosopher's stone, sought to find chemical remedies for diseases. Crude theories were supported by a few facts wrongly interpreted.

Early in the seventeenth century Glauber states that salt is the origin of all things. Boyle argues against the theory that "salt, sulphur, and mercury are the principles of things," and makes heat a powerful factor in originating new bodies. Becher thought that metals consisted of earth, of which there were three kinds—fusible or stony, fatty or fluid, and a "something of which they became deprived on ignition." This "something" Stahl named "phlogiston," which is akin to "spirits" and "souls" of the alchemists.

The phlogistic theory of Stahl was without foundation in fact, and yet, based upon experimental data, it was a step upward in chemical research, and held the minds of all for over one hundred and fifty years, including such great names in the eighteenth century as Hales, Black, Scheele, Priestley, Cavendish, and Lavoisier. Then it was that the analytic method became more accurate. Black, with the balance, demonstrated that the ignition of the metals magnesium and calcium gave no evidence that a ponderable "caloric" entered into them, but, to the contrary, a peculiar "fixed air" was expelled from them, which rendered them lighter than before they were burned.

The foundation of quantitative chemistry was thus laid, and the existence of "imponderable" agents in nature questioned. The discovery of "dephlogisticated air" by Priestley, the investigation of gases by Cavendish, of heat and fire by Scheele, and of insoluble minerals by Bergman—by means of the blow-pipe—were important additions to chemical knowledge, and enabled Lavoisier to generalize the facts already discovered. He announced a new theory of combustion, and, by questioning the existence of phlogiston, and showing that "principles should not be assumed where they could not be detected," revolutionized chemistry and gave it a new impulse, which has been quickened by every discovery since made.

Analysis of inorganic bodies increased, new facts accumulated, and new interpretations of phenomena were given, until the atomic theory, first suggested by Dalton in 1804, was promulgated under the great generalization known as the law of Avogadro or Ampère, which makes "equal volumes of all substances, when in a state of gas, and under like conditions, contain the same number of molecules."

This was the birth of modern chemistry, and, though it received attention when first enunciated in 1811, its far-reaching principles of truth were neither fully understood nor accepted for half a century afterward.

Chemistry, free from the errors of the past, now seeks to discover in the organic world the relations of different substances, as it has