071 Isomorphism, 409 



which it is intended to overcome is "very startling"; but 

 this is not all, for a theory still more destructive of what has 

 been considered as settled is propounded by the Professor, in 

 order that one difficulty in isomorphism may be got over, 

 where so many exist. 



After other hypothetical observations Professor Clark ob- 

 serves, " But partiality is unlike a law of nature, and indeed 

 the partiality disappears when we regard the oxygen salts as 

 having metals and not oxides for their bases." " According 

 to this view, when oil of vitriol, regarded as an hydrogen acid 



(H 2S) acts on an oxide, it is not a simple combination that 

 takes place, but a double decomposition, resulting in a neutral 

 salt and water, precisely as takes place when hydrochloric 

 acid acts on oxides;" and the Professor adds, " if observation, 

 which must be the final arbiter, shall determine one coinci- 

 dence more to accord with the doubled atoms of sodium and 

 silver, then, for aught I can see, the doctrine of oxygen salts 

 having oxides for their bases must be at once abandoned." 



Thus then because two salts are isomorphous which ought 

 not so to be, according to the received atomic weights of certain 

 bodies, these are not only to be altered, but we are on the 

 brink of losing altogether the numerous class of oxygen salts. 



It is however some consolation to observe that the cham- 

 pions of isomorphism differ so widely in their opinions, that 

 some chance remains of the permanence of the received doc- 

 trines as to atomic weights and the constitution of oxygen 

 salts. 



In the last Number of the Philosophical Magazine, p. 324, 

 Prof. Johnston has published a paper, the object of which is 

 to prove that the atomic weights of soda and silver, &c., in- 

 stead of being doubled to reconcile certain isomorphous dis- 

 crepancies, ought to be halved for the same cause. In other 

 words, the opinions of the Professors are as 4 to 1, on a sub- 

 ject in which though fancy has much to perform, observation 

 and facts ought to decide. 



Professor Johnston observes, " The isomorphism of two 

 compounds generally implies an analogy in their atomic con- 

 stitution,— that they are both sulphurets of the same order. 

 If the compound of copper be a disulphuret Cu, that of silver 

 is most probably a disulphuret Ag, and if so the atomic weight 

 of silver must be reduced one half, or to 6*75. We have there- 

 fore an argument in favour of the old result of Dulong and 

 Petit." 



This however is not all ; " the introduction of this change, 

 however," continues Professor Johnston, " would render ne- 



