CHEMICAL DIVISIBILITY. 41 



science has formed an opinion in regard to the relative weights of 

 these minute particles. The experiment referred to above may be so 

 conducted as to ascertain the weight of the products of decomposition 

 (viz., the oxygen and the mercury) of a given, previously weighed 

 quantity of oxide of mercury. In doing this, it will be found invari- 

 ably that every 13.5 parts by weight of the oxide of mercury yield 

 upon heating 12.5 parts by weight of mercury and 1 part of oxygen, 

 that we have consequently in 13.5 pounds of oxide 12.5 pounds of 

 mercury and 1 pound of oxygen. 



If we assume that a molecule of the oxide is composed of one atom 

 of mercury and one atom of oxygen, we are justified in saying that a 

 mercury atom is 12.5 times heavier than an oxygen atom. 



In a manner similar to this, the weights of the atoms of all different 

 elements have been compared with each other, and the element having 

 the lightest atom has been selected as the unit of comparison. The 

 element having the lightest atom is hydrogen, and we say the atomic ^ 

 weight of hydrogen is 1, and compare with this weight the weights of 

 all other elements. In doing this, we find that the atom of oxygen 

 weighs sixteen times as much as the atom of hydrogen, and we con- 

 sequently say the atomic weight of oxygen is 16. 



We have learned before, from the decomposition of the red oxide 

 of mercury, that the mercury atom is 12.5 times as heavy as that of 

 oxygen. As the atomic weight of this element is 16, the atomic 

 weight of mercury must be 12.5 times 16, or 200. 

 / Whilst atomic weight is the weight of the atom of any element as 

 / compared to the weight of an atom of hydrogen, molecular weight is 

 I the combined weight of the atoms forming the molecule. Thus the 

 I molecular weight of oxide of mercury is 200 + 16 = 216. 



Chemical symbols. For reasons to be better understood hereafter, 

 chemists designate each element by a symbol, and the first or first two 

 letters of the Latin name of the element have generally been selected. 

 Thus, the symbol of hydrogen is H, of oxygen O, of mercury Hg, 

 (from hydrargyrum), of sulphur S, etc. These symbols designate, 

 moreover, not only the elements, but one atom of these elements. 

 For instance : O not only signifies oxygen, but one atom or 16 parts 

 by weight of oxygen ; and Hg, one atom or 200 parts by weight of 

 mercury. 



Chemical formulas. In a similar manner as atoms of elements 

 are represented by symbols, the molecules of a compound substance 



