436 Doctor Clark to Professor Mitscherlich, 



and to be looked for, in the early history of a discovery so 

 pregnant with consequences. But there is a quality of iso- 

 morphous bodies that should arrest the attention of any 

 chemist, however devoted to the details and practice of 

 chemistry, and however averse to speculation. It is not, 

 that compounds differing- in their components, but alike in 

 constitution and form, approach each other so near in pro- 

 perties, as the phosphates and the arseniates, or as the 

 various individual alums are found to do, discordant as at 

 least some of such compounds manifestly are in the cha- 

 racter of their ultimate components ; but it is, that com- 

 pounds, when alike in constitution and form, although 

 different in elements, have, as was first established many 

 years ago by Beaudant and Wollaston, the property of 

 crystallizing together in proportions that are indefinite, yet 

 in such a manner as to produce crystals, perfect, in their 

 form, and as clear throughout their mass, as if they were 

 pure and unmixed, and with so little disturbance to the 

 harmony of the mixed compounds, as, whenever they bap- 

 pen to be of different colours, so to blend these as to pro- 

 duce tints, corresponding in depth to the proportions of the 

 various coloured compounds thaj: make up the crystalline 

 mixture.* Strange it is, to find slighted in importance, the 

 triumphant doctrine that, in achieving this discovery, 

 overthrew the last obstacle to the establishment of definite 

 proportions, and laid open to the chemist the mystery of 

 the mineral kingdom. 



While abiding by this your doctrine, and proceeding on 

 a like principle to what has just now been illustrated in the 

 case of the oxymanganate and the oxychlorate of potash, 

 it is possible, I conceive, to remove that unlikeness of con- 

 stitution, so apparent in the two following salts of like 

 form : — 



The oxymanganate of barytes Ba Mn Mn 



The waterless sulphate of soda . . . Na S 



These two salts, we may better compare, unswayed by any 

 theory, by regarding them in their ultimate components, 

 thus : — 



* This is the view taken of isomorphism by Dr. Thomson. See Inorganic 

 Chemistry, i. '21. — Edit. 



