DISCOVERY 



157 



Bolton thought they were produced by placing about 

 a quart in a bag and striking it, the sounds thus 

 elicited being heard at a distance of 450 feet.' 



Professor Bolton and Dr. Julien pubhshed a theory 

 to account for the cause of the phenomenon. They 

 believed that the individual grains of sand were 

 surrounded by films of condensed air, and that these 

 acted as elastic cushions, enabling the grains to vibrate 

 separately when disturbed. Thus they would behave 

 bke microphonolites when struck ; but there are many 

 insuperable objections to this theory. 



In 1886 I commenced my investigations with a view 

 to discovering the cause of the sounds, and was finally 

 led to beheve that they were due to the rubbing 

 together of millions of clean and incoherent grains 

 of quartz, with no angularities, roughness, or adherent 

 matter investing the grains ; and that, though the 

 vibrations emitted by the friction of any two grains 

 might be inaudible, those issuing from millions, 

 approximately of the same size, would give an audible 

 note. Working on this supposition, I produced in 

 189X notes from a treated sand that was not previously 

 musical.2 



The highly musical Eigg sand can be rendered mute 

 by adding a certain quantity of dust or purely angular 

 grains, and certain mute sands may become musical 

 by eliminating dust or angular grains. Again, com- 

 pletely spherical grains will not produce music. 



After all, it is the proportionate presence of the 

 different mineral fragments that decides the purity and 

 intensity of the notes, other conditions being favourable. 

 We can easily suppose that sand possessing all the 

 necessary conditions in great perfection will be most 

 musical, while those sands possessing all, or some, 

 in less degree, will emit sounds decreasing in musical 

 qualities in proportion to the decrement of the essential 

 factors, severally or in combination. 



Sound is the result of a series of pulsating waves of 

 air, or vibrations, which impinge upon the tympanum 

 or drum of the ear successively. When these vibra- 

 tions are irregular we get noise ; when they succeed each 

 other with sufficient rapidity, and at regular intervals 

 in a given period of time, the noise becomes music. 



If we take a sand composed of quartz and other 

 grains which are more or less angular, and vary con- 

 siderably in size, and strike it in a cup with a wooden 

 plunger, we get noise. But a musical sand composed 

 chiefly of quartz grains which are more or less rounded 

 and sub-angular, are well pohshed, and vary but 

 little in size, when struck in the same way produces 

 music. In the first case we get irregular vibrations, 

 and in the second case regular vibrations. 



' Some of my own observations and experiments at Eigg 

 were referred to in Nature, June 15, igri. 

 ' Nature, August 1891. 



iConiinued onp. i^ 



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