DIRECTION OF INDUCTIVE POLARITY— IN CRYSTALS. 271 



superior, gave me no indications of any difference in the inductive force for different 

 directions. 



1695. I then used cubes of Iceland spar. One 0*5 of an inch in diameter gave 100 

 for the axial direction, and 98*66 and 95*74 for the two cross directions. The other, 

 0*8 of an inch in the side, gave 100 for the axial direction, whilst 101*73 and 101*86 

 were the numbers for the cross direction. 



1696. Besides these differences there were others, which I do not think it needful 

 to state, since the main point is not confirmed. For though the experiments with 

 the first cube raised great expectation, they have not been generalized by those which 

 followed. I have no doubt of the results as to that cube, but they cannot as yet be 

 referred to crystallization. There are in the cube some faintly coloured layers parallel 

 to the optical axis, and the matter which colours them may have an influence ; but 

 then the layers are also nearly parallel to a cross direction, and if at all influential 

 should show some effect in that direction, which they did not. 



1697. In some of the experiments one half or one part of a cube showed a supe- 

 riority to another part, and this I could not trace to any charge the different parts had 

 received. It was found indeed that the varnishing of the cubes was sufficient to prevent 

 them receiving any charge, except (in a few experiments) a small degree of the nega- 

 tive state, or that which was contrary to the state of the inductric ball (1564. 1566.). 



1698. I think it right to say that, as far as I could perceive, the insulating cha- 

 racter of the cubes used was perfect, or at least so nearly perfect as to bear a com- 

 parison with shell-lac, glass, &c. As to the cause of the differences, other than 

 regular crystalline structure, there may be several. Thus minute fissures in the 

 crystal insensible to the eye may be so disposed as to produce a sensible electrical 

 difference (1193.). Or the crystallization maybe irregular: or the substance may 

 not be quite pure ; and if we consider how minute a quantity of matter will alter 

 greatly the conducting power of water, it will seem not unlikely that a little extra- 

 neous matter diffused through the whole or part of a cube, may produce effects 

 sufficient to account for all the irregularities of action that have been observed. 



1699. An important inquiry regarding the electrical polarity of the particles of an 

 insulating dielectric, is, whether it be the molecules of the particular substance acted 

 on, or the component or ultimate particles, which thus act the part of insulated con- 

 ducting polarizing portions (1669.). 



1700. The conclusion I have arrived at is, that it is the molecules of the substance 

 which polarize as wholes (1347.) : and that however complicated the composition of 

 a body may be, all those particles or atoms which are held together by chemical af- 

 finity to form one molecule of the resulting body, act as one conducting mass or par- 

 ticle when inductive phenomena and polarization are produced in the substance of 

 which it is a part. 



