14 W. J. MCGEE 



An important advance in chemistry was forecast in 1811 

 by the Italian Avogadro, and soon after by the Frenchman 

 Ampere, through the discovery that equal volumes of all sub- 

 stances, when in the gaseous state and under like conditions,, 

 contain the same number of molecules; that is, that the con- 

 stitution of matter is connected with its own inherent motion. 

 The discovery was barren until fertilized by the law of the 

 conservation of energy, and became fully fruitful only under 

 the skillful treatment of the American Cooke, who used it as 

 the basis of the new chemistry about the middle of the last 

 century. The advance marked the extension of natural law 

 into a field long cumbered by the mystical wreckage of alche- 

 my, and signalized the lifting of interpretation from the plane 

 of the material to that of the kinetic. A new chapter in the 

 history of chemistry was opened by Kekule, of Flanders, in 

 1858. This was the discover}^ of valence, or the law of pro- 

 portion under which atoms combine to form substances — a 

 far reaching, though possibly not final law governing the con- 

 stitution of matter. The laws of Avogadro and Kekule 

 yielded a larger view of the unknov/n; and by their aid Men- 

 delejeff, of Russia, and almost at the same time (1869-70) 

 Lothar-Meyer, of England, discovered that the known ele- 

 mentary substances fall naturally into groups displaying cer- 

 tain family resemblances, while the groups fall into series de- 

 fined by properties of the atoms; and these facts were formu- 

 lated in the remarkably comprehensive periodic law, or law 

 of Mendelejeff. 



From the culminating point of view afforded by this law 

 the domain of chemistry may be surveyed, as was the domain 

 of astronomy through aid of Kepler's law, and the endless 

 actions and reactions involved in the making and decomposi- 

 tion of materials, in growth and decay, are found to be no less 

 orderly and harmonious than the swing of satellites and planets 

 and suns in our solar and stellar systems; chemists can now 

 invade the unseen universe, and determine the proprieties of 

 elements not yet discovered, as Adams located Neptune by 

 formulas before it was detected by lenses. The power of 

 prevision possessed by chemists, under the periodic law, has 

 been established over and over again by successful predictions. 



