xix CHEMICAL NOMENCLATURE 273 



correct, i.e., sulphur dioxide may be represented by the formula 

 SO 2 . Sulphuretted hydrogen is also found to contain its own 

 volume of hydrogen, and by analogous reasoning we obtain SH 

 for its formula. 



Equivalent Weights and Valency. The methods employed 

 for finding the atomic weights differ in different cases. A method 

 of frequent application is to find the weight of an element which 

 t-an displace 1 gram of hydrogen from a compound, or which 

 unites with 8 grams of oxygen, or with 35 '5 grams of chlorine 

 (these being the weights which unite with 1 gram of hydrogen). 

 For example, it may be readily found that 12 grams of magne- 

 sium displace 1 gram of hydrogen from an acid, 1 or that 23 grams 

 of sodium displace 1 gram of hydrogen from water. These values 

 12 and 23 may be called the equivalent weights of magnesium 

 and sodium. It is, however, considered that while 1 atom of 

 sodium replaces 1 atom of hydrogen, yet the atom of mag- 

 nesium is capable of replacing 2 atoms of hydrogen. The atomic 

 weight of sodium is therefore 23, but in the case of magnesium 

 the atomic weight is 12 x 2, for we have found that the magne- 

 sium is 12 times heavier than the hydrogen it displaces, that is, 

 each atom is 12 times heavier than 2 atoms of hydrogen, i.e., 24 

 times the weight of 1 atom. The number of hydrogen atoms 

 which 1 atom of an element can replace is called its valency, 

 and from what has been said it will be seen that the atomic 

 weight = equivalent weight x valency. It would, however, take 

 us too far to discuss the methods of determining the valency, or 

 other methods of finding the atomic Aveights. 



Molecular Weights. It follows from Avogadro's Law that 

 the ratio of the weight of any volume of a gas to the weight of 

 the same volume of hydrogen, is equal to the ratio of the weight 

 of each molecule of the gas to the weight of the molecule of 

 hydrogen ; that is, the density of the gas referred to hydrogen 

 is the ratio of the molecular weight of the gas to the molecular 

 weight of hydrogen. Since the molecule of hydrogen contains 

 "2 atoms, its molecular weight is 2 ; hence 



Density referred to hydrogen = ^ f L wt of_gag_ = 



Mol. wt. of hydrogen 

 Mol. wt. of gas 



2 



i This experiment may easily be performed with the apparatus described in 



Expt. 296. 



