PHYSICAL SCIENCE IN THE NINETEENTH CENTURY 363 



that equal volumes of different gases under equal pressure change their 

 volumes equally with equal rise of temperature. These facts sug- 

 gested to Avogadro, and almost simultaneously to Ampere, the very 

 simple assumption that this is owing to the fact that equal volumes of 

 different gases contain an equal number of the smallest independent 

 particles of matter. This is Avogadro's celebrated hypothesis. It 

 was the first step in the direct physical verification of the atomic view 

 of matter. Merz. 



SYNTHESIS OF ORGANIC SUBSTANCES. Until the middle of the 

 nineteenth century there was an apparently fundamental separa- 

 tion between organic and inorganic nature. Since then they have 

 been brought together by the general laws of energy and to some 

 extent by the principles of evolution, as will appear in the following 

 chapter. In 1828 Wohler (of Gottingen) had indeed succeeded 

 in preparing urea out of inorganic materials, a discovery which 

 disproved such difference as was hitherto considered to exist 

 between organic and inorganic bodies. 



A PERIODIC LAW AMONG THE ELEMENTS. With gradually in- 

 creasing knowledge of the fundamental constants of chemistry 

 the atomic weights attempts were naturally made to connect 

 these with the chemical and physical properties of the correspond- 

 ing elements : valence, affinity, specific gravity, specific heat, etc. 

 In 1869-71 Mendelejeff, a Russian chemist, succeeded in establish- 

 ing remarkable relations between these data, and on tabulating 

 them enunciated his Periodic Law, which has resulted in the dis- 

 covery of several new and hitherto unsuspected elements. As the 

 existence of the planet Neptune (page 341) had been predicted to 

 fill an apparent gap in a system, so Mendelejeff under the periodic 

 law was able to predict the existence of other and missing ele- 

 ments in the series of chemical elements. And just as the pre- 

 diction of Adams and Leverrier was fulfilled by the actual discovery 

 of Neptune, so the prophecy of Mendelejeff was justified by the 

 discovery of gallium in 1871, scandium in 1879, and germanium 

 in 1886. Furthermore, the periodic law enabled Mendelejeff 

 to question the correctness of certain accepted atomic weights, and 

 here, also, he was justified by subsequent redeterminations. 



