172 COEKELATION OF PHYSICAL FORCES. 



more than one optic axis, the resultant of these axes points 

 diamagnetically. 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 ter- 

 restrial 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 exam- 

 ined, found to be crystalline in their structure : we thus get a 

 reciprocity of action between the force which unites the mole- 

 cules of matter and the magnetic force, and through the me- 

 dium of the latter the correlation of the attraction of aggre- 

 gation with the other modes of force may be established. 



I believe that the same principles and mode of reasoning 

 as have been adopted in this essay might be applied to the 

 organic as well as the inorganic world ; and that muscular 

 force, animal and vegetable heat, &c., might, and at some 

 time will, be shown to have similar definite correlations ; but I 

 have purposely avoided this subject, as pertaining to a depart- 

 ment of science to which I have not devoted my attention. 

 I ought, however, while alluding to this subject, shortly to 

 mention some experiments of Professor Matteucci, communi- 

 cated to the Royal Society in the year 1850, by which it ap- 

 pears that whatever mode of force it be which is propagated 

 along the nervous filaments, this mode of force is definitely, 

 aifected by currents of electricity. His experiments show 

 that when a current of positive electricity traverses a portion 

 of the muscle of a living animal in the same direction as that 

 in which the nerves ramify i. e. a direction from the brain 

 to the extremities a muscular contraction is produced in the 

 limb experimented on, showing that the nerve of motion is 

 affected ; while, if the current, as it is termed, be made to 

 traverse the muscle in the reverse direction, or towards the 



