The quantities of substances used in experiments on radio-activity are so small that they escape the ordinary methods of measurement, and are scarcely amenable to the ordinary processes of the chemical laboratory.

Fortunately, radio-activity can be detected and measured by electrical methods of extraordinary fineness, methods the delicacy of which very much more exceeds that of spectroscopic methods than the sensitiveness of these surpasses that of ordinary chemical analysis.
At the time of the discovery of radio-activity, about seventy-five substances were called elements; in other words, about seventy-five different substances were known to chemists, none of which had been separated into unlike parts, none of which had been made by the coalescence of unlike substances.

Compounds of only two of these substances, uranium and thorium, are radio-active.

Radio-activity is a very remarkable phenomenon.

So far as we know at present, radio-activity is not a property of the substances which form almost the whole of the rocks, the waters, and the atmosphere of the earth; it is not a property of the materials which constitute living organisms.