[12] The life-period of uranium is probably about eight
   thousand million years.

   [13] The life-period of thorium is possibly about forty
   thousand million years.

On p. 192 I said, that when the radio-active substances had been labelled elements, the facts of radio-activity led some chemists to the conclusion that the other bodies which had for long been called by this class-name, or at any rate some of these bodies, are perhaps not true elements, but are merely more stable collocations of particles than the substances called compounds.  It seems to me that this reasoning rests on an unscientific use of the term element; it rests on giving to that class-name the meaning, substances asserted to be undecomposable.  A line of demarcation is drawn between elements, meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts, and true elements, meaning thereby groups of identical undecomposable particles.  If one names the radio-active substances elements, one is placing in this class substances which are specially characterised by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the class.  To do this may be ingenious; it is certainly not scientific.

Since the time of Lavoisier, since the last decade of the eighteenth century, careful chemists have meant by an element a substance which has not been separated into unlike parts, and they have not meant more than that.  The term element has been used by accurate thinkers as a useful class-mark which connotes a property—­the property of not having been decomposed—­common to all substances placed in the class, and differentiating them from all other substances.  Whenever chemists have thought of elements as the ultimate kinds of matter with which the physical world is constructed—­and they have occasionally so thought and written—­they have fallen into quagmires of confusion.

Of course, the elements may, some day, be separated into unlike parts.  The facts of radio-activity certainly suggest some kind of inorganic evolution.  Whether the elements are decomposed is to be determined by experimental inquiry, remembering always that no number of failures to simplify them will justify the assertion that they cannot be simplified.  Chemistry neither asserts or denies the decomposability of the elements.  At present, we have to recognise the existence of extremely small quantities, widely distributed in rocks and waters, of some thirty substances, the minute particles of which are constantly emitting streams of more minute, identical particles that carry with them very large quantities of energy, all of which thirty substances are characterised, and are differentiated from all other classes of substances wherewith chemistry is concerned, by their spontaneous mutability, and each is characterised by its special