-274 On Crystallography* 



The nomenclature in all the foregoing expressions, - 

 as well as in those which follow, makes an abstraction 

 oF tlie faces parallel to those of the nucleus, which ex- 

 ist most frequently in the secondary crystal. Among 

 the forms in which the nucleus is entirel) disguised, 

 some liave names borrowed from different consi- 

 derations; and those which remain are so few in num- 

 ber, that I thought it unnecessary to complicate the 

 language by enijiloying a paniculardesignation for them. 

 In order to avoid confounding the words which 

 express the decrements with those which indicate the 

 number of the faces, we may remark, that the former 

 have their termination in hedral, as dodecahedral, or in 

 «/, as octodecagonal, whereas the others end in ary. 



c» Equivalent, when the part visible {exposant) which in- 

 dicates a decrement isrqual to the sum oF those which 

 indicate the others. Ex. Equivalent sulphated iron. 



/. Suhtrociive^ when the part visible relative to a decre- 



' ment is less by unity than the sum of those which in- 

 dicate the others. Ex. Subtractive pyroxene. 



a. Additive^ when the part visible relative to a decrement 

 exceed'-: by unity the sum of those vi-hich indicate the 

 others. Ex. Additive sulphated copper. 



h. Progressive, when the parts visible forni a commence- 

 ment (if arithmetical progression; as 1, 2, 3. Ex. 

 Progressive tourmaline. 



i. Disjoif/tcd, when the decrements form an abrupt leap, as 

 from 1 to 4 or to 6. Ex. Disjointed sulphurated an- 

 timoniated silver. 



k. Partial, when there is some part which remains without 

 decrements, while the other parts similarly situated 

 undergo them. Ex. Partial sulphurated cobalt. <t . 



/. Suhdouhie, when the part visible relative to a decrement 

 is the half oF the sum of the other parts visible. Ex. 

 Subdouble topaz. 



We say subtriple, sub quadruple, &c. in the same 

 way. Ex. Subtriple sulphated copper. 



The three parts visible {exposans) which compose 

 the indication of an intermediary decrement, count as 

 one only, which is equal to their sum. 



m, Doiddiiig, tripling, quadruplingy when one of the vi- 

 sible parts is repeated twice, thrice, or four times in 

 one series which would otherwise be regular, Ex. 

 Doubling peridot, quadrupling peridot. 



//. Identical, when the parts visible of the simple decrements, 

 lo the number of two, are equal to the terms of the 



fraction 



