OTHER MODES OF FORCE. 155 



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

 motion. Thus, when arrested motion produces heat, it matters 

 not whether the motion has been produced by a falling body, 

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

 pound, &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 

 consider there is a different sort of link. 



Mosottihas mathematically treated of the identity of gravi- 

 tation with cohesive attraction, and Pliicker has succeeded in 

 showing that crystalline bodies are definitely affected 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 influence 

 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 more 

 than one optic axis, the resultant of these axes points dia- 

 magnetically. The mineral cyanite is influenced by mag- 

 netism in so marked a manner that when suspended it will 

 arrange itself definitely with reference to the direction of 

 terrestrial magnetism, and may, according to Pliicker, be used 

 as a compass-needle. 



There is scarcely any doubt that the force which is con- 

 cerned in aggregation is the same which gives to matter its 

 crystalline form ; indeed, a vast number of inorganic bodies, 

 if not all, which appear amorphous, are, when closely examined, 

 found to be crystalline in their structure : we thus get a re- 

 ciprocity of action between the force which unites the mole- 

 cules of matter and the magnetic force, and through the 

 medium of the latter the correlation of the attraction of 

 aggregation with the other modes of force may be established. 



I believe that the same principles and mode of reasoning 



