Case of Isomorphous Substitution. 407 



plesiomorphous binary compounds) be equally accidental, or 

 rather are not all these governed by some other at present 

 unknown law ? Can you throw any light upon this difficult 

 subject. Yours truly, 



H. J. Brooke. 



LXVII. Observations on Isomorphism^ in reference to the 



precedi?ig Communication by Mr. Brooke. By R. Phillips, 



F,n.S, L, 4" Ed. 

 /^N comparing the analyses of zoiziteand euclase quoted in 

 ^^ Mr. Brooke's letter, it is evident that substituting glucina 

 for lime, these minerals may be considered as similar ; but 

 whether this be a case of isomorphous substitution I now 

 propose to inquire. 



Professor Johnston ( Report of British Association, vol. i. 

 p. 423) states that " binary compounds which replace each 

 other contain not only the same absolute number of atoms, 

 but also the atoms of the two elements in the same relative 

 proportion." According to Berzelius the equivalent of lime 

 is 28*53, and the (double) equivalent of glucina is 77*13; 

 then 28-53 : 21 : : 77*13 : 56*77, the quantity of glucina which 

 on this supposition should replace 21 of lime: if we take the 

 single equivalent of glucina the quantity of it contained in 

 euclase would, of course, be 28*38 ; but the actual quantity is 

 only 21*78, or about three fourths of what it should be, on the 

 supposition most favourable to the doctrine of isomorphism. 



But another difficulty presents itself in considering lime 

 and glucina as isomorphous ; Prof. Johnston observes, in the 

 paper above quoted, " if in peroxide of iron the iron is to the 

 oxygen in the atomic ratio of two to three, the atoms of alu- 

 minum and oxygen must in alumina have the same ratio, or 

 both bases must be sesquioxides." Similar reasoning will, I 

 presume, apply to other oxides which are supposed to be iso- 

 morphous ; now, according to Berzelius, lime is a protoxide, 

 while glucina is a sesquioxide; these substances, therefore, 

 cannot be isomorphous. 



There is however every reason to believe that this, and 

 indeed any difficulty whicB is presented to the doctrine of 

 isomorphism, will be readily overcome, when we observe the 

 liberties which the expounders of it take with what appear 

 to be the best established facts. 



In the Records of Science (vol. iii. p. 433) there is a paper 

 by Professor Clark, in which, on account of " a difficulty in 

 isomorphism," he has proposed to double the atomic weights 

 of certain substances. Addressing Professor Mitscheriich, 

 Professor Clark states : 



