52 Dr. Clark to Professor Mitscherlich, 



Thus, in its consequences, threatening to shake what che- 

 mists have been accustomed to consider as most fixed, the dif- 

 ficulty in isomorphism that I have pressed upon your atten- 

 tion, is not, like some former difficulties, one of mere detail, 

 rectified, perhaps, by adverting to the w^ater of crystalliza- 

 tion, and, when rectified, leaving unaffected all the other 

 details, and all former views of chemistry. Here, on the 

 contrary, is a difficulty concerning a point, upon which, 

 when granted, the world of chemical doctrine may be moved. 

 Proportional to the importance of such a point at issue, will 

 be the caution of chemists in scrutinizing the stability of 

 the evidence. On a single point, indeed, however well 

 established, few men will be disposed to rest all the conse- 

 quences that the one at issue may be destined to bear. Even 

 Archimedes, it may be suspected, had he, in answer to the 

 enthusiasm of his wish, obtained that one stable point he 

 desired, would, in the moment for action, have sighed for 

 another. Content, therefore, with depicting to chemists 

 the consequences of this difficulty, I leave the issue to be 

 determined, as it can only be, by some future instance, 

 equally unequivocal, of coincident form and constitution in 

 compounds of sodium or silver, compared with compounds 

 of barium, or strontium, or lead, or calcium, and, perhaps, 

 I might add other metals. Such coincidence, in respect of 

 constitution, will accord, it may be supposed, either with 

 the received atoms of sodium and silver, or with those atoms 

 doubled. That the coincidence shall prove according to 

 the received atoms of those metals, is rendered little pro- 

 bable by the fact, that, according to that standard, many 

 coincidences in constitution are already known, without 

 any coincidence in form having been yet observed ; whilst, 

 according to those atoms doubled, scare any crystalline 

 compounds of entirely coincident constitution are as yet 

 known.* If observation, which must be the final arbiter, 



* Supposing that the present atomic weiglit of sodium should be doubled, the 

 following formulas would represent, according to the received constitution of 

 oxygen-salts, and to the "present atomic weights, some salts of soda and of barytes 

 that might prove of coincident form : — 



NaA Ba A A 



Na A Ba A A 



Na A Ba A A 



