824 PRINCIPLES OF CHEMISTRY 



motion and the distances between the individual molecules, or of the 

 atoms in the molecules, or of their sum total, but they do not alter the 

 original equilibrium of the system ; whilst chemical changes, on the 

 other hand, alter the molecules themselves, that is, the velocity of 

 motion, the relative distribution, and the quality and quantity of the 

 atoms in the molecules. 



Atoms are the smallest quantities or chemically indivisible masses 

 of the elements forming the molecules of elements and compounds. 



Atoms have weight, the sum of their weights forms the weight of 

 the molecule, and the sum of the weights of the molecules forms the 

 weight of masses, and is the cause of gravity, and of all the phenomena 

 which depend on the mass of a substance. 



The elements are characterised, not only by their independent exist- 

 ence, their incapacity of being converted into each other, <fec., but also 

 by the weight of their atoms. 



Chemical and physical properties depend on the weight, composi- 

 tion, and properties of the molecules forming a substance, and on the 

 weight and properties of the atoms forming the molecules. 



This is the substance of those principles of molecular mechanics 

 which lie at the basis of all contemporary physical and chemical 

 constructions since the establishment of the law of Avogadro- 

 Gerhardt. The fecundity of the principles enunciated is seen at 

 every step in all the particular cases forming the present store of 

 chemical data. We will here cite a few examples of the application of 

 the law. 



As the weight of an atom must be understood as the minimum 

 quantity of an element entering into the composition of all the mole- 

 cules formed by it, therefore, in order to find the weight of an atom of 

 oxygen, let us take the molecules of those of its compounds which have 

 already been described, together with the molecules of certain of those 

 carbon compounds which will be described in the following chapter : 



Molecular Amount of 



Weight Oxygen 



H 2 O 18 16 



NijO 44 16 



,N'O 30 16 



KOo 46 32 



Molecular Amount of 



Weight Oxygen 



HN0 3 63 48 



CO 28 16 



CO 2 44 32 



The number of substances taken might be considerably increased, 

 but the result would be the same that is, the molecules of the com- 

 pounds of oxygen would never be found to contain less than 16 parts 

 by weight of this element, but always n!6, where n is a whole number. 



