158 Dr. S. Tolver Preston on some 



particles is explained quite as completely as the air-pressure 

 is explained by the encounters of such particles (molecules) . 

 If one only accepts as valid the two principles of the conser- 

 vation of energy and of the centre of mass, then one must 

 attribute elasticity as well to the a?ther as to gas molecules, 

 without being troubled about the further explanation of its 

 nature. 



Therefore I have without hesitation regarded molecules of 

 open structure as elastic, which implies that by the impact of 

 such molecules no energy is annihilated. Dr. Isenkrahe 

 regards the molecules of bodies as absolutely hard solid 

 spheres which, in order that gravity by atomic encounters 

 (i. e. its proportionality to mass) may be explained, must be 

 far apart from each other. How can one imagine to oneself 

 a structure composed of perfectly hard molecules situated far 

 apart which shall have only tolerable stiffness and stability? 

 Such a body made up of widely separated spherical molecules, 

 if no other forces but gravity acted, could at the most behave 

 like a gas, but never as a solid or liquid body. 



On the other hand, elastic molecules of open structure may 

 be made to cohere at their boundaries by the pressure of the 

 smaller atoms, which at the same time easily fly through the 

 open parts of the structure. Have we not here at least a 

 groundwork for the conceptions upon which we may hope 

 to build further? 



Dr. Isenkrahe gives no limits for the value of the mean 

 length of path, whereas it seems to me to be a very important 

 point of my theory, that the mean length of path must be 

 assumed great in comparison with the planetary distances. 



Concerning the calculations which Dr. Isenkrahe attaches 

 to his theory, Dr. A. M. Bock, who made the theory of 

 Isenkrahe the basis of his 'Inaugural Dissertation'*, expresses 

 himself as follows : — 



" The aim and the purpose of the atomic aether theory, 

 namely to construct universal gravitation, is, as mentioned in 

 the introduction to the Rathsel von der Schwerkraft, not fully 

 attained. There is no formula deduced from which, as a 

 starting-point, one could follow out the theory further. 

 One sees oneself forced therefore, in the sense of the theory, 

 to deduce an (if only in some measure rigorous) expression 

 for the attraction " (p. 18, under the paragraph-title Die 

 Anziehung zweier Korper). 



On the developments and modifications which Dr. Isen- 

 krahe's calculations received through Dr. Bock I allow myself 



* Wolf & Sohn, Munich, 1891. 



