142 ON CRYSTALLOGRAPHY. 



It t us rather wait with patience till future discoveries enable us to 

 ire farther ; and satisfy ourselves in the mean time with ar- 

 ranging those laws of affinity which have been ascertained, with, 

 out deriding u liether it be the same force with gravitation, or a 

 different one. [Thomson. 



CHAP. XI. 



ON CRYSTALLOGRAPHY. 

 rri 



1 HE word crystal (x/w<r7aA>of) originally signified ice; but it 

 was afterwards applied by the ancients to crystallized silica, or 

 rock crystal ; because, as Pliny informs us, they considered that 

 body as nothing else than water congealed by the action of cold. 

 Chemists afterwards applied the word to all transparent bodies of a 

 regular shape; and at present it is employed to denote, in general, 

 the regular h'gure which bodies assume when their particles have 

 full liberty to combine according to the laws of cohesion. These 

 regular bodies occur very frequently in the mineral kingdom, 

 and have long attracted attention on account of their great 

 beauty and regularity. By far the greater number of the salts as. 

 sume likewise a crystalline form ; and as these substances are 

 mostly soluble in water, we have it in our power to give the regu- 

 lar shape of crystals in .some measure at pleasure. 



1. Most solid bodies either occur in the state of crystals, or are 

 capable of being made to assuma that form. Now it has long 

 been observed by chemists and mineralogists, that there is a par. 

 ticular form which every individual substance always affects when 

 it crystallizes : this indeed is considered as one of the best marks 

 fur distinguishing one substance from another. Thus common salt 

 is observed to assume the shape of a cube, and alum that of oclahe. 

 dron, consisting of two four-sided pyramids, applied base to base. 

 Saltpetre affects the form of a six-sided prism ; and sulphate of 

 magnesia that of a four-sided prism ; and carbonate of lime is of- 

 ten found in the state of a rhomboid. Not that every individual 

 substance always uniformly crystallizes in the same form ; for this 



