248 THE SCIENTIFIC MONTHLY
weight of the atom compared with the atom of hydrogen. One of the most important means for arriving at the number of atoms in the mole- cule^ and in that way the atomic weighty is by determining the velocity of sonnd in the gas. This velocity is found in a very ingenious manner in a quantity of gas contained in a glass tube a few feet long and a small fraction of an inch in diameter. The determination of the atomic weight of an element is one of the most important investigations connected with it and for these elements physical not chemical means must be used.
Belonging to the same group of elements is still another gas^ niton, which is not found in measurable quantity in the air. It is given off by the element radium and is sometimes called radium emanation. That it is not found in the air is not to be wondered at, since if a quantity equal to that of argon in the atmosphere were suddenly introduced into the air, it would within three months diminish to less than the quantity of xenon changing from one part in a hundred to one part in two hundred million of air. This is because niton decomposes so rapidly. In less than four days any quantity will dimiuiRh to one half what it was at the beginning of the time. One of the products of decomposition is helium, another is a substance metallic in character which itself readily disinte- grates. This substance deposits on a negatively charged body brought iato contact with niton, and the fact that a negative wire in the atmos- phere acquires such a deposit, which may be rubbed off or dissolved by ammonia, is taken as uidication of the presence iQ the air of an infin- itesimal quantity of niton. Any further discussion of this matter would lead too far afield.
The above are all the elements Jenoton to belong to this group ; but the Bussian chemist Mendel^f, whose arrangement of the elements ac- cording to their atomic weights in series and in groups was epoch-making in the science of chemistry, suggests that there may be two other ele- ments in the group, elements very much lighter than hydrogen, one of them almost infinitely lighter. One is the corona of the sun, the other the luminiferous ether. At present we have no means of testing Mendelief s hypothesis.
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