OTHEE MODES OF FOECE. 171 



and with no resisting medium, then, as long as that rotation 

 continues, the motion of the meteoric mass itself would be 

 the exponent of the force impelling it ; if there be a resist- 

 ing medium, part of this motion would be arrested and taken 

 up by the medium, either as motion, heat, electricity, or some 

 other mode of force ; if the meteor approach the earth suffi- 

 ciently to fall upon it, the perceptible motion of the meteor 

 is stopped, but is taken up by the earth which vibrates 

 through its mass ; part also reappears as heat in both earth 

 and meteor, and part in the change in the earth's position 

 consequent on its increase of gravity, and so on. Gravita- 

 tion is but the subjective idea, and its relation to other modes 

 of force seems to me to be identical with that of pressure or 

 motion. Thus, when arrested motion produces heat, it mat- 

 ters not whether the motion has been produced by a falling 

 body, i. e. by gravitation, or a body projected by an explo- 

 sive compound, &c. ; the heat will be the same, provided the 

 mass and velocity at the time of arrest be the same. In no 

 other sense can I conceive a relation between gravitation and 

 the other forces, and, with all diffidence, I cannot agree with 

 those who seek a more mysterious link. 



Mosotti has mathematically treated of the identity of 

 gravitation with cohesive attraction, and Pliicker has recently 

 succeeded in showing that crystalline bodies are definitely af- 

 fected by magnetism, and take a position in relation to the 

 lines of magnetic force dependent upon their optical axis or 

 ,axis of symmetry. 



What is termed the optic axis is a fixed direction through 

 crystals, in which they do not doubly refract light, and which 

 direction, in those crystals which have one axis of figure, or 

 a line around which the figure is symmetrical, is parallel to 

 the axis of symmetry. When submitted to magnetic influ- 

 ence such crystals take up a position, so that their optic axis 

 points diamagnetically or transversely to the lines of magnetic 

 force ; and when, as is the case in some crystals, there is 



