90 HISTORY OF CHEMISTRY. [LECTURE VI. 



in the Latin nomenclature (which Berzelius proposes to 

 employ) ; for example, oxidum ferrosum (with the smaller ratio 

 of oxygen) and oxidum ferricum. Finally, superoxides are also 

 distinguished. These contain a relatively large proportion of 

 oxygen, and must be reduced before they form salts. 



What Berzelius says respecting compounds with water, is of 

 interest. According to him, water may occur in compounds, 

 combined in three different ways. It plays the part, either of 

 an acid, as in the caustic alkalies, or of a base, when it unites 

 with acids. In both of these cases it is called water of hydra- 

 tion, and is distinguished from water of crystallisation which 

 unites with salts, and can be separated from these again, without 

 their being, thereby, essentially changed in their nature. 



The system of notation of Berzelius is original with himself, 

 and, up to the present, it has always proved so thoroughly 

 practical that we have retained his proposals almost without 

 alteration. In his system, the atom of an element is represented 

 by the initial letter of the Latin name of the element ; and by 

 placing the symbols side by side, the atoms of compounds are 

 obtained. When several atoms of one element occur in a 

 compound, a figure giving the number of these is placed after 

 the letter, and above (or below) the line. The so-called double 

 atoms (i.e., two atoms of an element, which occur together) 

 constitute an exception to this; in these the symbol for the 

 atom is " barred." 13 Thus, for example, H = H 2 , or two atoms 

 of hydrogen ; HO = H 2 O, or one atom of water, consisting of 

 two atoms of hydrogen and one of oxygen, etc. 



In the cases of more complicated compounds, several letters 

 are separated from others by the sign + ; and the mode of 

 division is dependent on the dualistic view. For the sake of 

 shortness, the atom of oxygen is often represented by a point, 

 and that of sulphur by a vertical stroke ; and in compounds, 

 these marks are placed above the symbol of the element with 

 which the oxygen or sulphur is combined a method of writing 

 formulae that has now, however, been abandoned. 



13 Lehrbuch. 3, part I, 108. 



