764 THE POPULAR SCIENCE MONTHLY. 



are possible only -when facts have been observed, and definite knowl- 

 edge 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 dis- 

 covery of " dephlogisticated air " by Priestley, the investigation of 

 gases by Cavendish, of heat and fire by Scheele, and of insoluble min- 

 erals by Bergman — by means of the blow-pipe — were important addi- 

 tions 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 Ampere, which 

 makes " equal volumes of all substances, when in a state of gas, and 

 under like conditions, contain the same number of molectdes." 



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 



