ONE HUNDRED YEARS OF CHEMISTRY 305 



and similar discrepancies might appear in nearly all 

 formulas containing elements of different valencies. In 

 1858, however, an article by the Italian chemist Canniz- 

 zaro appeared in which the outlines of a course in chem- 

 ical philosophy were presented. This acquired wide 

 circulation in the form of a pamphlet at a chemical con- 

 vention somewhat later, and it dealt so clearly and ably 

 with Avogadro's principle, Dulong and Petit 's law, and 

 other points in connection with formulas that it led to a 

 rapid and almost universal reform among those who 

 were using unsatisfactory formulas. 



At about this time also the dualistic formulas of Ber- 

 zelius were generally abandoned, and hydrogen came to 

 be regarded as the characteristic element of all acids. 

 For instance, CaO.S0 3 , called "sulphate of lime," came 

 to be written CaS0 4 and was called "calcium sulphate," 

 and while it had been shown as early as 1815 by Davy 

 that "iodic acid," I 2 5 , showed no acid reaction until it 

 was combined with water, the accumulation of similar 

 facts led to the formulation of sulphuric acid as H 2 S0 4 

 instead of S0 3 or H 2 O.S0 3 , and that of other "oxygen 

 acids" in a similar way. As a necessary consequence of 

 this view of acids, the bases came to be regarded as com- 

 pounds of the "hydroxyl" group, OH. Therefore the 

 formula for caustic soda came to be written NaOH 

 instead of Na 2 O.H 2 0, and so on. 



The Periodic System of the Elements. The perio- 

 dicity of the elements in connection with their atomic 

 weights was roughly grasped by Newlands in England, 

 who announced his "law of octaves" in 1863. This was 

 at the time when the atomic weights were being modified 

 and their numerical relations properly shown. The sub- 

 ject was worked out more fully by L. Meyer in Germany 

 a little later, but it was most clearly and elaborately pre- 

 sented by the Russian chemist Mendeleeff in 1869. 



In order that this subject may be explained to some 

 extent Mendeleeff 's table is given here, with the addition 

 of the recently discovered elements and some other mod- 

 ifications. 



