LAWS AND THEORIES OF CHEMISTRY. 99 



We may define molecules as the smallest particles of matter that 

 can exist in the free state and still retain all the properties of the 

 substance to which they belong. If we divide the molecule, we 

 destroy the properties of the substance as we know it in the free state. 



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 15.88 parts 

 by weight of oxygen ; and Hg, one atom or 198.5 parts by weight of 

 mercury. 



Symbols or formulas of compounds. By suitable experiments 

 it is possible for the chemist to determine not only the kinds of ele- 

 ments in a compound and their proportions, but also how many atoms 

 are in a molecule of the substance. For the sake of economy of 

 time and space and to give a better insight into their nature, sub- 

 stances are represented by formulas, which is a shorthand way of 

 telling what would require many words. A formula primarily rep- 

 resents the composition of a molecule, but as any quantity of a sub- 

 stance is simply the sum of a great multitude of molecules all. alike, 

 the formula, in a broader sense, stands for the substance itself. Thus 

 we say HgO is mercuric oxide, although it shows the composition of 

 a molecule of the oxide, namely, that it consists of one atom of mer- 

 cury and one atom of oxygen. 



The formulas are formed by writing the symbols of the constituent 

 elements side by side, and the number of atoms of each element, 

 when more than one atom is present, is represented by a figure below 

 and to the right of the symbol of the element ; thus H 2 O means that 

 a molecule of water consists of two atoms of hydrogen and one atom 

 of oxygen ; CO 2 means that the carbon dioxide molecule consists of 

 one atom of carbon and two atoms of oxygen. A number placed before 

 a formula signifies so many molecules, thus 2H 2 O means two mole- 

 cules of water. As the formulas represent the size of the molecules, 

 they are called molecular formulas, and the sum of the atomic weights 

 of all the atoms composing the molecules is called the molecular 

 weight, which shows how many times heavier the molecule is than an 

 atom of hydrogen. 



