between the Earthy Carbonates and Alkaline Nitrates. 483 



510 such relation can be detected between the parts or simple 

 compound bodies of which they are made up ; on the con- 

 trary, the wider our knowledge extends, the more numerous 

 in all probability will such instances become. 



According to this reasoning, an analogy in atomic consti- 

 tution among the several constituents, simple or compound, 

 which make up any pair of isomorphous bodies, chemically 

 different, is not a condition w^c^ssarj/ to their existence. A 

 general analogy, however, may be necessary between the iso- 

 morphous bodies taken as a whole, whether that of an equal 

 ratio between the positive and negative equivalents present in 

 them, similar to that made out as above between the earthy 

 carbonates and the alkaline nitrates ; or some other relation, 

 as that between the several elements in the isomorphous 

 crystals of permanganate of baryta and anhydrous sulphate of 

 soda, to which the attention of chemists has lately been drawn 

 by Dr. Clarke*. 



Still as deductions from observation we know but few ex- 

 ceptions to the law of Mitscherlich, that identity of form im- 

 plies analogy of constitution in compound bodies as well as 

 identity of form in both, and analogy of constitution in one at 

 least of the parts or members of which they are composed. 



It may hereafter be established, that an equality in the num- 

 ber of positive and negative equivalents, or even an equality 

 in the ratio of these, may give the power to crystallize in the 

 same form, or in the same suite of two or more forms ; but 

 though such views derived from reasoning may enable us to 

 explain certain anomalies in this and other branches of the 

 science, yet we must carefully distinguish them from that cer- 

 tain knowledge which is derived from direct observation, re- 

 garding them only as a portion of that dawning light which 

 precedes the establishment of every great principle. 



In the above observations therefore I would be understood 

 as desirous rather of bringing the matter more under the at- 

 tention of chemists, and familiarizing them with the mode in 

 which such relations as that observed between the earthy car- 

 bonates and alkaline nitrates may be supposed to take place, 

 than as advocating the reception of the hypothesis I have 

 stated, which further observation may prove to be wholly de- 

 stitute of foundation. 



Durham, Sept. 1837. 



* See Dr. Clarke's letter to Mitscherlich, in the Records of General 

 Science, 



2S2 



