1902.] on the Electronic Theory of Electricity. 167 



to reconstruct, in the light of newly acquired knowledge, our scientific 

 theory of any group of effects. Thus, the whole of electrical phe- 

 nomena have become illuminated of late years by a theory which has 

 been developed concerning the atomic structure of electricity, and this 

 hypothesis is called the Electronic Theory of Electricity. 



The Atomic Theory. 



The opinion that matter is atomic in structure is one which has 

 grown in strength as chemical and physical knowledge has progressed. 

 From Democritus, who is said to have taught it in Greece, to John 

 Dalton who gave it defiuiteness, and to Lord Kelvin who furnished the 

 earliest numerical estimate of the size of atoms, in spite of adverse 

 criticism, it has been found to be the best reconciler of very diverse 

 and numerous observed effects. Let us consider what it really means. 

 Suppose we take some familiar substance, such as common table salt, 

 and divide a mass of it into the smallest grains visible to the eye. 

 Each tiny fragment is as much entitled by all tests to be called table 

 salt, or to give it the chemical name, sodic chloride, as a mountain of 

 the material. Imagine that we continue the subdivision under a good 

 microscope ; we might finally obtain a little mass of about one hundred- 

 thousandth of an inch in diameter, but beyond this point it would 

 hardly be visible even under a powerful lens. We may, however, 

 suppose the subdivision continued a hundredfold by some more deli- 

 cate means until we finally arrive at a small mass of about one ten- 

 millionth of an inch in diameter. A variety of arguments furnished 

 by Maxwell, Boltzmann, Loschmidt, Lord Kelvin and others show 

 that there is a high degree of probability that any further subdivision 

 would cause the portions into which the salt is divided to be no 

 longer identical in properties, but there would be two kinds of parts 

 or particles, such that if all of one kind were collected together they 

 would form a metal called sodium, and if all of the other kind were 

 similarly picked out they would form a non-metal called chlorine. 

 Each of these smallest portions of table salt, which if divided are no 

 longer salt, is called a molecule of sodic chloride, and each of the 

 parts into which the molecule is divisible is called an atomy of 

 sodium or of chlorine. In dealing with the dimensions of these very 

 small portions of matter an inch or a centimetre is too clumsy a unit. 

 To express the size of an atom in fractions of an inch is worse than 

 stating the diameter of an apple in fractions of a mile. Every one 

 knows what is meant by a millimetre ; it is nearly one twenty-fifth 

 part of an inch. A metre is equal to a thousand millimetres. 

 Suppose a millimetre divided into a thousand parts. Each of 

 these is called a micron and denoted by the Greek letter jx. This 

 however is still too large a unit of length for measuring the size 

 of atoms, so we again divide the micron into a thousand parts and call 

 each a micromillimetre or micromil, and denote it by the symbol fijx. 

 Lord Kelvin's estimate of the diameter of a molecule is that it lies 



