THE MYSTERY OF MATTER. 177 



Chemists recognize some 80 or more elements, which combine 

 with each other in certain definite and unvarying proportions. 

 Thus two parts or atoms of hydi-ogen combine with one part of 

 oxygen to form a molecule of water. From these combining 

 proportions what are called the atomic weights of the elements 

 have been deduced, that is, the propoi-tional weights of the 

 different atoms. These atomic weights vary very much, ranging 

 from hydrogen, which is represented by 1, to uranium, which is 

 designated as 238 ; an atom of uranium being 238 times as 

 heavy as an atom of hydrogen. The specific gravities or 

 densities of the elements, though greater in elements with high 

 atomic weights, do not regularly increase with increase in the 

 weight of the atoms, showing us that some atoms must be 

 heavier or more massive in proportion to their size than others. 

 Atoms have been supposed to be indivisible and indestructible, 

 but recent discoveries have tended materially to modify our 

 opinion on these points. Atoms combine with each other to 

 form molecules, and molecules are the smallest portions of any 

 substance which can exist as such. 



Molecules and atoms are all exceedingly minute. It has been 

 calculated that a cubic centimetre of air contains 21 trillions of 

 molecules, and that it takes 10 trillions of molecules of air 

 (10 millions of millions of millions) to make a milligramme ; 

 and that a milligramme of hydrogen would contain 144 trillions 

 of molecules ; so that their minuteness is absolutely beyond all 

 conception. The diameter of a molecule of gold has been 

 calculated not to exceed the five millionth part of a millimetre, 

 and it may be considerably less. The smallest organic speck, 

 which is visible in the most powerful microscope, is in diameter 

 about the four thousandth part of a millimetre, and it is 

 considered that such a speck contains at least two millions 

 of organic molecules, which, as we have seen, are much larger and 

 more complex than molecules of an inorganic nature. And such 

 a tiny speck must contain at least one hundred millions of atoms. 

 All molecules and atoms of the same substance are exactly 

 similar to each other in every respect. 



Molecules even in a solid body are not in contact, but are 

 situated at some distance from each other, the intervening spaces 

 being occupied by ether. They are continually oscillating to 

 and fro in very rapid motion, each molecule having, so to speak, 

 its own orbit of motion. If we apply heat to a solid body, such 

 as a crystal of some salt, we get first an increase in the motion 

 of the component molecules, causing elevation of temperature ; 



