THE ATOMIC VIEW OF NATURE. 



445 



nomenon has been called isomorphism. The discovery 

 has been of great practical value, as well as theoretical 

 interest. If the definite and invariable form of existence 

 which the crystal exhibits is considered as a proof of the 

 purity of a chemical substance, and if in the same crystal 

 one elementary substance can be replaced by one or several 

 other substances, then this substitution must take place in 

 definite proportions of weight, in the equivalent propor- 

 tions. Thus the production of such isomorphous crystals 

 affords a method of determining the relative atomic 

 weights or equivalents. As such it was hailed by Ber- 

 zelius ; the more so, as in no case did the equivalents 

 thus obtained contradict the numbers he had found by 

 other methods. 1 Theoretically, the property of isomor- 

 phism acquired a still greater interest when Mitscher- 



1 In the early days of the atomic 

 theory as developed by Berzelius, 

 great uncertainty existed as to the 

 numbers which were to be chosen for 

 the atomic weights of the elements. 

 This was owing to the property of 

 fixed multiple ratios it remaining 

 undecided which was the smallest 

 submultiple of a given combining 

 ratio in which any special element 

 could enter into combination. 

 Other methods were then used to 

 assist in deciding this point. The 

 law of volumes, and later the pro- 

 perties of isomorphism, were there- 

 fore hailed by Berzelius as welcome 

 aids in fixing the atomic numbers. 

 Both these methods are still used, 

 though the latter is not always de- 

 cisive. The most important method 

 according to the present state of our 

 knowledge is the determination of 

 the vapour density, where such can 

 be got, and that of the specific heat 

 in the solid state. It is mainly 

 owing to Cannizzaro (1858) that 



the apparent contradictions, which 

 were supposed to exist in the num- 

 bers arrived at by various methods, 

 were explained by reverting to 

 Avogadro's forgotten hypothesis. 

 The periodic law or arrangement of 

 the elements into classes showing 

 similar physical properties is like- 

 wise of use. A complete, lucid, 

 and exhaustive statement of the 

 most recent position of our know- 

 ledge of the true atomic weights 

 of the elements will be found in 

 Lothar Meyer's posthumous tract, 

 ' Die Atome und ihre Eigenschaf- 

 ten,' Breslau, 1896. In this valuable 

 book, as also in Ostwald's ' Allge- 

 meine Chemie,' vol. i. , will also be 

 found an account of the degree of ac- 

 curacy which attaches to our present 

 knowledge of the atomic and com- 

 bining numbers, which form the 

 solid foundation of all quantitative 

 chemistry and all practical applica- 

 tions. 



