There are some ten thousand simple tones in the complete tonal scale; but the compound tones employed by music are only about a hundred in number, and are selected from a middle range of hearing. The compound tone, as we have said, breaks up on analysis into simple partial tones; the lowest is called the =fundamental=, the others the =overtones=. It is a remarkable fact that the overtones always stand in a definite relation to the fundamental. The various musical instruments do not, however, sound all the overtones alike; their construction favours some, and weakens or destroys others; and that is the main reason why we can tell a harp-tone, for instance, from a tone of the same pitch played on oboe or trumpet. The compound tones thus owe their =colour= or =timbre=, in the first instance, to _the number and relative loudness of the overtones which accompany the fundamental_. Timbre has other factors; but this is the primary source of difference. _Overtones may readily be heard._ Strike a _c_, very lightly, on the piano. When it has ceased to sound, strike loudly the _c_ next below; you can probably, even at the first trial, hear the higher _c_ in the lower. Now strike very lightly the _g_ next above your higher _c_, and then the lower _c_ again loudly; you will probably hear the _g_. Helmholtz, working with thin strings, was able to hear no less than fifteen overtones with the fundamental. This blending of the partial tones in a compound tone, to give a single and unitary impression, is an example of what is called =tonal fusion=. The best fusion is that of two tones which constitute an octave; here, indeed, the blend is so close that it is often confused with unison; a soprano and a bass singer, told to sing in unison, will start off without hesitation an octave apart. Next after the octave stands the fifth (_c_ and _g_); boys who think they are whistling the same notes often whistle, in fact, a fifth apart. Other pairs of tones give lesser degrees of fusion. _Tones generate as well as blend._ If you sound together two high tones, such as you get from a double bicycle whistle, or from small bottles of different sizes, you hear, besides these tones themselves, a third tone, very much deeper, larger, more booming; this =differential tone= is easy to find and, once heard, cannot be mistaken. Only, the two tones must not be too nearly alike in pitch; for, if they are, you hear, instead of a differential tone, slow surges or quick rattlings of sound. Take two bottles of the same size, and mistune one of them by pouring in small amounts of water; have them blown steadily together; the course of the =beats=, as they are called, from a slow surge through a rattle to a harsh blur, may thus be followed.