LAWS OP CHEMICAL COMBINATION. NOTATION. 31 



by fermentation, or by the explosion of gunpowder, always 

 possesses the same definite composition. This is one of the 

 four great laws governing chemical combination, and is 

 known as the Law of Definite Proportions. Before taking 

 up the study of the other laws you must understand the 

 meaning of chemical symbols. 



28. Chemical Symbols. Now turn to the table on page 

 13, and we will tell you how to use it. First you have a list 

 of all the elementary substances, and opposite each name 

 you find letters placed, which are either the first two letters 

 of the full name or some other abbreviation, the meaning of 

 which you will now learn. These letters are the symbols 

 used so much in chemistry, and we will explain some of them. 

 Take those elements whose names begin with S. We call 

 sulphur S, and we must therefore designate the others in 

 such a way as to distinguish them from this and from each 

 other. "We call, then, silver Ag, from Argentum, the Latin 

 for silver ; and sodium N"a, from an old Latin name for soda, 

 Natrium. Phosphorus is set down as P, being a prominent 

 element with a name beginning with that letter. Then Po- 

 tassium is designated by K, from JTalium, an old Latin 

 name for potash. Several of the elements take their sym- 

 bols from their Latin names. Thus the Latin for Antimony 

 is Stibium, Sb ; for Mercury, Hydrargyrum, Hg; Lead, 

 Plumbum, Pb ; Copper, Cuprum, Cu ; Gold, Aurum, Au ; 

 Iron, Ferrum, Fe ; and Tin, Stannum, Sn. Generally those 

 which are most important are represented by a single let- 

 ter; as, for example, the four grand elements, oxygen, ni- 

 trogen, carbon, and hydrogen. We use these symbols as a 

 kind of short-hand for expressing chemical reactions, and it 

 is necessary to become familiar with this method of writ- 

 ing from the very outset. Thus, instead of writing Potas- 

 sium in full, we write K, or in place of Hydrogen we write 

 H. This is not all they signify, however : each letter stands 



