14(5 On Crystallography. 



two ranges, which only acts hi the same way on the ridge* 



conliguoas to the lo\ycr summit. Finally, the quantities e 



and E ouglit to be thus considered independently of eack 

 other ; the first as expressing a decrement by two ranges 

 on the angles e solely, and the second as indicating zero of 

 decrement, on the angles E opposite to the foregoing. 



I have enlarged on the detail of the principles of the me- 

 thod, in order to leave nothing to be desired, if it were possi- 

 ble, of what can be of use in enabling my readers to have a 

 clear idea of the art, and put an observer in possession of 

 the method of instantly representing a secondary crystal of 

 a given form. But if any person confined himself to the 

 simple comprehension of the signs employed in the system, 

 and was only anxious to read without being able to write 

 them, he would only require some simple and easily under- 

 stood rules, which we shall here succinctly explain ; — they 

 will form a kind of recapitulation of the preceding details. 



1 . Every vowel employed in the sign of a crystal designates 

 the solid angle marked with the same vowel on tl>e figure 

 which represents the nucleus ; and every consonant indi- 

 cates the ridge which bears this same consonant, or the 

 face the middle of which it occupies. 



2. Every vowel or every consonant is accompanied by a 

 cypher, the value as well as the position of which indicates 

 the law of decrement which the corresponding angle or edge 

 undergoes. We must except the three consonants P, M, T, 

 each of which, when it forms part of the sign of a crystal, 

 indicates that this crystal has faces parallel to that which 

 bears this same letter. 



3. Every letter comprehended in the sign of a crystal i& 

 marked below with ihe cypher that accompanies it, on all 

 tho angles or all the edges which perform the same function 

 with that which on the figure is marked immediately with 

 the letter in question. 



4. Every number added to a letter indicates a decre- 

 ment, the angle or edge of which marked with this letter is 

 the term of departure. If the number be entire, it indicates 

 iiow many rows are subtracted in breadth, with the condi- 

 tion that every lamina has only the thickness of one mole- 

 cule ; if the nmiiber be fractionary, the numerator makes 

 known how many rows are subtracted in breadth, and the 

 denominator how many are subtracted in height. 



5. According as the number is placed below or above 

 the letter which it accompanies, it indicates that the decre- 

 ment 



