172 CORRELATION 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 Plucker, 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- 

 affected 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 



