A HALF-CENTURY OF SCIENCE. 2 i\ 



regard the entire material universe as a gigantic engine, and that after 

 long use we have exhausted all the fuel (in its most general sense) in 

 the world ; then all the energy available will have become dissipated, 

 and we shall have arrived at a condition of things from which there is 

 no apparent escape. 



Professor Frankland has been so good as to draw up for me the 

 following account of the progress of chemistry during the last half- 

 century : Most of the elements had been discovered before 1830, the 

 majority of the rarer elements since the beginning of the century. In 

 addition to these the following five have been discovered, three of 

 them by Mosander, viz. : lanthanum in 1839, didymium in 1842, and 

 erbium in 1843. Ruthenium was discovered by Claus in 1843, and 

 niobium by Rose in 1844. Spectrum analysis has added five to the 

 list, viz. : caesium and rubidium, which were discovered by Bunsen 

 and Kirchhoff in 1860 ; thallium, by Crookes in 1861 ; indium, by 

 Reich and Richter in 1863 ; and gallium, by Lecoq de Boisbaudran 

 in 1875. 



As regards theoretical views, the atomic theory, the foundation of 

 scientific chemistry, had been propounded by Dalton (1804-1808). 

 The three laws which have been chiefly instrumental in establishing 

 the true atomic weights of the elements the law of Avogadro (1811), 

 that equal volumes of gases, under the same conditions of temperature 

 and pressure, contain equal numbers of molecules ; the law of Dulong 

 and Petit (1819), that the capacities for heat of the atoms of the 

 various elements are equal ; and Mitscherlich's law of isomorphism 

 (1819), according to which equal numbers of atoms of elements be- 

 longing to the same class may replace each other in a compound with- 

 out altering the crystalline form of the latter had been enunciated in 

 quick succession ; but the true application of these three laws., though 

 in every case distinctly stated by the discoverers, failed to be generally 

 made, and it was not till the rectification of the atomic weights by 

 Cannizzaro, in 1858, that these important discoveries bore fruit. 



In organic chemistry the views most generally held about the year 

 1830 were expressed in the radical theory of Berzelius. This theory, 

 which was first stated in its electro-chemical and dualistic form by its 

 author in 1817, received a further development at his hands in 1834, 

 after the discovery of the benzoyl-radical by Liebig and Wohler. In 

 the same year (1834), however, a discovery was made by Dumas, which 

 was destined profoundly to modify the electro-chemical portion of the 

 theory, and even to overthrow the form of it put forth by Berzelius. 

 Dumas showed that an electro-negative element, such as chlorine, 

 might replace, atom for atom, an electro-positive element like hydro- 

 gen, in some cases without much alteration in the character of the 

 compound. This law of substitution has formed a necessary portion 

 of every chemical theory which has been proposed since its discovery, 



