Page:Popular Science Monthly Volume 59.djvu/601

Rh the molecular weight of argon), through 1°, allowing it to expand while it is being heated, is 5 calories. Deducting the 2 calories required for external work, 3 calories remain as the heat required to raise the temperature of the gas. For oxygen, on the other hand, the heat required to raise the temperature of 32 grams, its molecular weight, through 1° is 7 calories; and deducting 2 as before, the remainder is 5, the specific heat of oxygen. Hence for argon and for oxygen, we have the properties:

The argument stands thus: The heat required to raise the temperature of argon without expansion can be accounted for entirely on the supposition that it is wholly used in causing the molecules to move through space; on the other hand, more heat requires to be communicated to oxygen than to argon in the proportion of 3 to 5. With oxygen and similar gases, this extra heat must be doing something; it is supposed to produce motion of the atoms within the molecule. There is no such motion within the argon molecule; hence it is concluded that the molecule consists of a single atom; and in. that case, the molecular weight is the same as the atomic weight. The molecule of oxygen may be considered as possessing a structure like that of a dumb-bell; the atoms forming the knobs at each end of the bar. On throwing a dumb-bell through space, it will not merely change its position as a whole; but it will rotate. But a molecule of argon or helium is imagined to have the simpler form of a sphere or ball; when it is thrown practically no energy is used in causing it to rotate, but it is all expended in making it pass through space.

I must apologize for introducing such abstruse conceptions into a popular exposition; but they are necessary to the argument; and I am afraid that no simpler means can be found of reaching the conclusion that the molecules of argon and of helium are identical with their atoms.

As 4 is the molecular weight of helium, and as 40 is that of argon, these numbers also stand for their atomic weights. Let us next see how these figures fit into the periodic table.

In 1897, as president of the Chemical Section of the British Association, I chose the title An Undiscovered Gas' for the address to the Section. The arguments in favor of the existence of such a gas were briefly these: The differences between the atomic weights of consecutive elements in the columns of the periodic table are approximately 16 to 20; thus 16.5 is the difference between the atomic weights of fluorine and chlorine; 16, between those of oxygen and sulphur, and so on.