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.