2i 4 HERMANN VON HELMHOLTZ 



two papers you announce on the cooling of the earth, and the 

 alterations of form in elastic spherical shells, which may con- 

 ceivably have reference to the earth also, since I am now 

 engaged on some lectures to the students of all faculties on 

 the general results of natural science, in which I want to give 

 a popular exposition of the law of the conservation of energy 

 and its consequences, and to use it as a connecting thread to 

 draw the different branches of the physical sciences together. 

 I have so far given the history of the planetary system of the 

 sun and earth, and convinced myself that many problems have 

 been neglected by the astronomers and geologists, which might 

 well be attacked now, under the present conditions of know- 

 ledge, though only by those who are trained physicists and 

 mathematicians. Your undertaking to write a Textbook of 

 Natural Philosophy is very praiseworthy, but will be exceed- 

 ingly tedious. At the same time I hope it will suggest ideas 

 to you for much valuable work. It is in writing a book like 

 that, that one best appreciates the gaps still left in science. 



' My book on acoustics is just out, with the title Die Lehre 

 von den Tonempfindungen, als physiologische Grundlage fur die 

 Theorie der Musik (On the Sensations of Tone, as a Physiological 

 Basis for the Theory of Music). The publisher has already 

 informed me that the copy I intended for you has been sent 

 to your address in Glasgow. The issue of this book and the 

 pro-rectorial business that has devolved on me this year have 

 taken up so much of my time that I have not been able to 

 attend to anything else. Now I have gone back to the com- 

 pletion of my Physiological Optics, one section of which is still 

 wanting/ 



In his Sensations of Tone Helmholtz set himself the task 

 of connecting the border-land of physical and physiological 

 acoustics with that of the science of music and aesthetics, so 

 that he leaves out of consideration pure physical acoustics, 

 which is merely a part of the theory of elastic bodies. 

 Following the principle he had adopted for the work on 

 optics, he divides his physiological acoustics into three parts, 

 the first of which is occupied with determining the mode in 

 which sound is conducted to the sensory nerve within the ear, 

 and contains the physical part of the corresponding physio- 

 logical investigation of the sensations ; the second, and more 



