100 



CHEMISTRY. 



stand for representations of what is real is at- 

 tested by the fact that the most successful ex- 

 perimenters of the day confess to having heen 

 guided by them at almost every step of their re- 

 searches. A modern constitutional formula 

 takes the various atoms of a compound in the 

 proportions indicated by the empirical formula, 

 and in the absolute number prescribed by the 

 molecular weight, and arranges them in the 

 way which, within the limits of the law of 

 valency, will best account for the reactions of 

 the compound. The doctrine of these formulas 

 originated in the difficulty of explaining isom- 

 erism under Dalton's law of atomic weights, and 

 of indicating it by his empirical formulas. The 

 phenomena of isomerism did not militate 

 against the theory of atoms, but really gave it 

 support; for while they were incongruous with 

 the other theories of matter, they were sus- 

 ceptible of explanation under the hypothesis 

 that it consists of atoms variously arranged. 

 From this point of view it was necessary to 

 determine, so far as possible, the mode of ar- 

 rangement of the atoms in the different com- 

 pounds. The results of this attempt are em- 

 bodied in the constitutional formula?. The 

 method resorted to in solving this problem was 

 similar to that which had been employed in 

 determining the ultimate composition of com- 

 pounds. Just as when, after isolating from a 

 compound, or introducing into a compound, 

 some particular kind of elementary matter, 

 chemists concluded that the compound actually 

 contained that kind of matter, so when in a 

 reaction a particular group of atoms was elim- 

 inated bodily from a compound, or introduced 

 bodily into a compound, they concluded that 

 this group existed as such in the compound. 

 This conclusion, however, is not always so 

 warrantable in the case of atomic groups as in 

 the case of elements. The reaction, for exam- 

 ple, by which an atomic group is eliminated 

 from a compound involves the destruction of 

 the parent compound, and in the process it is 

 easy for the atomic groups to undergo rear- 

 rangement. The development of the idea of 

 the molecule as distinct from that of the atom 

 and the discovery of a means of determining 

 the molecular weight of bodies, led to the divis- 

 ion of isomerides into two classes those in 

 which the proportions of the various atoms 

 were the same, but the number of atoms in the 

 molecule was different, constituting cases of 

 polymerism ; and those in which both the 

 proportions and the whole number of the 

 atoms were the same, or isomeric bodies proper. 

 The knowledge of the molecular weight aided 

 in the construction of constitutional formula 

 by determining the exact number of atoms in 

 the molecule, and thus facilitating the task of 

 arranging those atoms by stating precisely how 

 many had to be arranged. The law of valency 

 exercised an important influence by limiting 

 the number of legitimate arrangements. The 

 validity of a constitutional formula is attested 

 by its correctly predicting reactions which 



were not contemplated during its construc- 

 tion. It is not probable that there is any- 

 thing like the constitutional formula in the 

 sense of its being a picture in the molecule 

 itself, but quite possibly there is something 

 corresponding with it. A comparison of the 

 physical properties of similarly-constituted 

 compounds shows that in many cases very 

 definite relations can be traced between con- 

 stitution and physical properties. They are 

 found in such features as the melting-point, 

 boiling-point, specific gravity, behavior toward 

 light, and molecular volumes at boiling-points. 

 Sir Henry E. Roscoe devoted his presiden- 

 tial address before the British Association at 

 Manchester, chiefly to the general discussion 

 of the present development and prospects of 

 the science of chemistry. Dalton's theory 

 of atoms had become established after fifty 

 years of research, and that had been accom- 

 plished, at least to a certain extent, respecting 

 it which he had declared impossible the at- 

 oms had been measured. Whatrare the exact 

 relations between the atomic weights of the dif- 

 ferent elements whether or not they are mul- 

 tiples of some ultimate unit has still escaped 

 painstaking investigation. But it appears to 

 have been determined that u while the atomic 

 weights are not exactly either multiples of the 

 unit or of half the unit, many of the numbers 

 expressing most accurately the weight of the 

 atom, approximate so closely to a multiple of 

 that of hydrogen, that we are constrained to 

 admit that these approximations can not be a 

 mere matter of chance, but that some reason 

 must exist for them ; what that reason is, and 

 why a close approximation, and yet something 

 short of absolute identity exists, is as yet hid- 

 den behind the veil." Another set of rela- 

 tionships between the elements and their at- 

 omic weights has engaged Lothar Meyer, 

 Mendelejeff, and Carnelly in the investigation 

 from which they have agreed in placing all 

 the elementary bodies in a regular sequence, 

 and thus bringing to light a periodic recurrence 

 of analogous chemical and physical properties. 

 Under this system chemists have ventured to 

 predict the atomic weights and qualities of un- 

 discovered elements, and in several cases 

 gallium, scandium, and germanium these pre- 

 dictions have been fulfilled. Such results, al- 

 though they by no means furnish a proof of 

 the supposition that the elements are derived 

 from a common source, clearly point in this 

 direction. Evidence regarding this inquiry is 

 furnished by the action of heat, and consists 

 in the fact that distinct chemical individuals 

 capable of existing at low temperatures, are 

 incapable of existence at high ones, but split 

 up into new materials possessing a less com- 

 plicated structure than the original. Not only 

 compound bodies have been decomposed by 

 this method, but Victor Meyer has proved in 

 the case of iodine, that the molecule which 

 the author defines as a more or less compli- 

 cated aggregation of atoms is broken into 



