MOLECULAR PHYSICS. 79 



now clearly manifests itself to consider the various physical forces 

 as being all varieties of the mechanical force, differing from one 

 another in method of application and in their effects upon the 

 nature of the bodies on which they act The thrust through a 

 stick, the blow of a wave of sound, the attraction of a magnet, the 

 push of a gas when expanded by heat, are all mechanical forces, 

 and, therefore, it is argued with much reason, mechanical force 

 must be in activity in the space between the active and passive 

 body. Hence the general conception that the force which holds 

 together the parts of a bar of steel is due to the same cause as 

 that which urges the bar to the ground, as that which forces it 

 towards a magnet, and as that which causes it to rust in moist air. 

 From the fact that, under like conditions, chemical union occurs 

 in constant weight-ratio, arose the first modern conception of 

 atoms as being indivisible, invisibly small, presumably spherical, 

 masses of the elements. It was concluded that each of the atoms 

 of which one and the same element consists has the same weight, 

 and that the atoms of other elements have all other different 

 weights, and that the weight-ratio of chemical combinations is com- 

 pounded of the weight-ratio of the elementary atoms, and of their 

 relative number. According to the atomic theory, the uniformity 

 of composition of the body water, every nine pounds of which 

 consists of one pound of hydrogen and eight pounds of oxygen, 

 depends upon (a) the fact that when water is formed every atom 

 of oxygen unites with one atom of hydrogen, joined with the fact 

 that the oxygen atom Is eight times as heavy as the hydrogen ; or 

 (b) that each atom of oxygen unites with two atoms of hydrogen, 

 joined with the fact that the atom of oxygen is -sixteen times as 

 heavy as that of hydrogen. Such a hypothetical group of atoms 

 is called a molecule, and it is generally supposed that such 

 molecules may, without undergoing internal derangement, act 

 like atoms in combining with atoms or other molecules. It has 

 been maintained by some that the atoms 111 an element are never 

 free, but that they are grouped together or combined as molecules, 



