390 J. P. Cooke on the Numerical Relation 
doubtedly, crystalline form; but a classification of the elements 
based solely on the principles of isomorphism is defective in the 
same way as it isin mineralogy. It brings together, undoubtedly, 
allied elements, but it also groups with them those which resem- 
ble each other only in their crystalline form. The mode of com- 
bining seems to be also a fundamental property; but, like crys- 
talline form, it would bring together in some instances elements 
differing very widely in their chemical properties. A classifica- 
tion of the elements which shall exhibit their natural affinities, 
ments is hardly possible in the present state of chemistry. At 
best, the task is attended with great difficulties, and it cannot be 
expected that these should be surmounted at once. The c assifi- 
eation which is offered in this memoir will, undoubtedly, ‘be 
found to contain many defects. If, however, it is but one step 
to the science. It was originally made, as has already been Sale; 
simply for the purpose of teaching, and never would have been 
published had it not led to the discovery of the numerical rela- 
it 
chemistry. In the group of volatile acids, homologues of formic 
acid, for example, we have a series of compounds yielding § 
derivatives, and producing similar reactions, and many of whose 
properties, such as boiling and melting points, specific gravity, 
vary as we descend in the series according to a determina 
* 
te 
From formic acid, a highly limpid, volatile, and corrosive fluid, 
. 
* 
