250 ARSENIATED COPPER. 



arfeniated x:opper. You feem to confider as nothing the very 

 fenfible differences which exift in the divers fpecies which I 

 have eftabliflied, with refpe6t to hardnefs, fpecific gravity, and 

 colour ; and, flopping at the fingle charader of form, you make 

 fuppofitions for each of them, which, in fact, terminate by 

 conneding all thofe which they offer with one primitive cryr 

 fial : but nature does not exhibit any of the decrements which 

 you fuppofe. I have never difcovered the flighteft trace of 

 them in any of the immenfe quantity of cryftals of arfeniated 

 copper which have pafied through ray hands. Do you believe 

 that thefe fuppofitions are only fufceplible -of being admitted 

 in a cafe in which, all the other characters being agreed in the 

 mod perfed (late of thefe fubftances, which is that of regular 

 cryftallization andtranfparency, they would become neceflary 

 only to add an accumulation of proofs to thofe already ac- 

 quired of their identity. 

 IxzQ. diftinc- Never was more attention paid thaji at this moment to the 



tion of the fpe- g^gat truth, that the progrefs of the fciences which lead to the 

 the progrefs of ftudy of nature, depends principally on the exad diftindion of 

 the fcience. each of (he fpecies whofe union forms the aggregate to which 

 the fcience is applied. No one is more convinced of this im- 

 portant truth than I am. But this exa6t knowledge of the 

 fpecies, which perhaps your calculation or the analyfis of im- 

 proved chemiftry may one day attain in a fimple and accurate 

 manner, refls at prefent on the agreement of the exterior fpe- 

 cific charadlers. Whenever tliis agreement exifts^ we are? 

 compelled to conclude that there is a fimilitude in the fpecies, 

 and, on the contrary, a ditfimilarity when they differ eflen- 

 lially from each other. I, however, agree perfectly with you, 

 that before feparating one of thefe fubftances from the other 

 to make a fpecies of each, it is requifite to he previoufly con- 

 vinced that the differences which they offer, and on which their 

 divifion refls, are not purely accidental. It appears to me, 

 therefore, that nothing can be more undeferving of the re- 

 proach of having negledied thefe precautions, than, on the 

 contrary, the eftablidiment of the divifion on the invariable 

 conflancy in the difference of their exterior characters. 

 Chemical analy- The only reafon, which, in the fubftances in queftion, can 

 fis does not j.^j|-^. . doubt on tiieir difference, is the refult obtained from 



fuperkde the. •' . 



mineralogkal them by chemical analyfis, which conftantly found the arfeni- 

 cliaraiters, ^al acid cornbincd with the copper in each of them ; but if the 



analy lis 



