As hydrogen is the lightest of the elements, it has been a favorite theory with scientists that the various elements are all composed of combinations of hydrogen atoms. But since many of the elements have atomic weights which cannot be made exact multiples of that of hydrogen, it has been felt that there must be some other smaller unit than the hydrogen atom; or else that these hydrogen atoms themselves change in weight when they combine to form other atoms. But mass seems to be the one unchangeable characteristic of matter; hence it was felt that any change of weight is almost unthinkable, and so a solution was sought in the direction of still further dividing the hydrogen atom, the smallest unit concerned in chemical change, as then understood. But now the facts and principles brought to light in connection with the studies of radioactivity have settled it that we actually do have a much smaller unit than the hydrogen atom, one of only about 1/1760 its mass, in fact; and that this smallest of the small things of nature is none other than a particle of negative electricity, now called an _electron_. That the atoms of all the elements must have a common unit of composition, that they behave as if composed of ultimate particles that may be regarded as duplicates of one another, has long been regarded as an inevitable conclusion from the Periodic Law of Mendeleef. This law says that the physical as well as the chemical properties of the various elements depend upon their atomic weights, or as it is stated in the language of mathematics, the properties of an element are functions of its atomic weight. This fact of the variation in the properties of elements in accord with their atomic weights has been even more strikingly illustrated by the behavior of discharges of electricity through rarified gases, as well as by the facts of radioactivity. To quote the words of Sir J.J. Thompson, "The transparency of bodies to Roentgen rays, to cathode rays, to the rays emitted by radioactive substances, the quality of the secondary radiation emitted by the different elements, are all determined by the atomic weight of the element."[1] [Footnote 1: Encyclopædia Britannica, Vol. XVII, 891. Cambridge Edition.]