We may here refer to another new conception to which the discovery of
radio-activity has given rise. Lord Kelvin, who estimated the age of the
earth at twenty million years, reached this estimate by considering the
earth as a body which is gradually cooling down, "losing its primitive
heat, like a loaf taken from the oven, at a rate which could be
calculated, and that the heat radiated by the sun was due to
contraction." Uranium and radio-activity were not known to Kelvin, and
their discovery has upset both his arguments. Radio-active substances,
which are perpetually giving out heat, introduce an entirely new factor.
We cannot now assume that the earth is necessarily cooling down; it may
even, for all we know, be getting hotter. At the 1921 meeting of the
British Association, Professor Rayleigh stated that further knowledge
had extended the probable period during which there had been life on
this globe to about one thousand million years, and the total age of
the earth to some small multiple of that. The earth, he considers, is
not cooling, but "contains an internal source of heat from the
disintegration of uranium in the outer crust." On the whole the estimate
obtained would seem to be in agreement with the geological estimates.
The question, of course, cannot, in the present state of our knowledge,
be settled within fixed limits that meet with general agreement.
[Illustration: MAKING THE INVISIBLE VISIBLE
Radium, as explained in the text, emits rays--the "Alpha," the "Beta"
(electrons), and "Gamma" rays. The above illustration indicates the
method by which these invisible rays are made visible, and enables the
nature of the rays to be investigated. To the right of the diagram is
the instrument used, the Spinthariscope, making the impact of radium
rays visible on a screen.
The radium rays shoot out in all directions; those that fall on the
screen make it glow with points of light. These points of light are
observed by the magnifying lens.
A. Magnifying lens. B. A zinc sulphite screen. C. A needle on whose
point is placed a speck of radium.