344 On Crystallography, 



correct to say that they belong exclusively to a certain 

 $pecies of mineral, in such a manner that the idea which 

 they originate will be the faithful rejiresentation. 



The character in question will be placed at the head of 

 those which compose the specific character. It would, 

 perhaps, be pushing the matter too far, to require that it 

 should always distinguish precisely the substance to which 

 it is applied, not only from all those of the same class, but 

 in general from all the minerals. It would seem to be 

 allowable to understand the classical name from its repre- 

 sentative (enoncS)^ in such a way as to form with this name 

 the entire definition of the substance for which ii has been 

 chosen. In this manner \vc define telesia, an earthy sub- 

 stance with a specific gravity of about 4, and which only 

 presents well defined joints perpendicularly to the axis of 

 Its crystals. We shall see, however^ that in a great number 

 of cases the essential character taken by itself gives an ex- 

 dusion to all the minerals different from that which it de- 

 signates. 



To conclude : — ^I do not flatter myself with having al- 

 ways succeeded in making the best possible <:hoice of the 

 characters which ought to form that which I call essential; 

 and they will be found sometimes a little vague, when they 

 refer to substances of which we have as yet but a slight 

 notion. Time will add to our stock of knowledge, and 

 this will serve to give more edge to those parts of the pic- 

 ture which are too feebly marked in the present state of 

 science. 



Still, however, this was not enough ; and one of two 

 things may have happened. Either the observer, who 

 wished to determine a mineral, would proceed straight for- 

 ward to the species of which this mineral formed a part, 

 and then he would have nothing else to do but to consult 

 the essential and specific characters, to ascertain that he 

 was right; or, deceived by a false resemblance, he would be 

 led 10 a foreign species. To bring him back to the right 

 path in this last case, we have added, at the end of the 

 specific character, another which we call distinctive, com- 

 posed of the principal differences which may enable us tq 

 pick ■out a mineral froqn among those with which we should 

 be tempted to confound it. 



We have also placed at the head of each class a general 

 view of the substance^ which it contains, with the enume- 

 ration of the characters, the assemblage of which may serve 

 to distinguish this class from the others ; and we have en- 

 deavoured \o restrict llieSe characters, §o that there may 



not 



