710 KEPOBT— 1893. 



established fact that a relation exists between the power which a dissolved chemical 

 compound possesses of producing the colour impression within our comparatively 

 small visual range, and the particular mode of grouping of its constituent radicals in 

 its molecule. Further, the reality of this connection will be most freely admitted 

 in the -class of aromatic compounds ; that is, in derivatives of benzene, whose con- 

 stituents are so closely linked together aa to exhibit quasi-elemental persistence. 

 If, then, the possession of what we call colour by a compound be connected with 

 its constitution, may we not infer that ' elements ' which exhibit distinct colour, 

 such as gold and copper, m thin layers and in their soluble compounds, are at least 

 complexes analogous to definitely decomposable substances ? This inference, while 

 legitimate as it stands, would obviously acquire strength if we could show that any- 

 thing like isomerism exists among the elements ; for identity of atomic weight of 

 any two chemically distinct elements must, by all analogy with compounds, imply 

 dissimilarity in constitution, and, therefore, definite structure, independently of 

 any argument derived from colour. Now, nickel and cobalt are perfectly distinct 

 elements, as we all know, but, so far as existing evidence goes, the observed difler- 

 ences in their atomic weights (nickel 58'G, cobalt 58'7) are so small as to be within, 

 the range of the experimental errors to which the determinations were liable. Here, 

 then, we seem to have the required example of something like isomerism among- 

 elements, and consequently some evidence that these substances are complexes^ 

 of difierent orders; but in the cases of cobalt and nickel we also know that in 

 transparent solutions of their salts, if not in thin layers of the metals themselves,. 

 they exhibit strong and distinct colours — compare the beautiful rosy tint of cobalt 

 sulphate with the brilliant green of the corresponding salt of nickel. Therefore,. 

 in exhibiting characteristically difierent colours, these substances aftbrd us some 

 further evidence of structural diflerences between the matter of which they consist, 

 and support the conclusion to which their apparent identity in atomic weight, 

 would lead us. By means of such side-lights we may gradually acquire some idea 

 of the nature of the elements, even if we are unable to get any clue to their origin 

 other than such as maj' be found in Crookes' interesting speculations. 



Again, while our knowledge of the genesis of the chemical elements is as smalL 

 as astronomers possess of the origin of the heavenly bodies, much suggestive work 

 has recently been accomplished in the attempt to apply the principle of gravitation,. 

 which simply explains the relative motions of the planets, to account for the inter- 

 actions of the molecules of the elements. The first step in this direction was sug- 

 gested by Mendeleef in his Royal Institution lecture (May 31, 1889), wherein he 

 proposed to apply Newton's third law of motion to chemical molecules, regarded a» 

 systems of atoms analogous to double stars. The Rev. Dr. Ilaughton has- 

 followed up this idea with his well-known mathematical skill, and, in a series of 

 papers just published, has shown that the three Newtonian laws are applicable to 

 explain the interactions of chemical molecules, ' with this ditl'erence, that whereas 

 the specific coefficient of gravity is the'same for all bodies, independent of the particu- 

 lar kind of matter of which they are composed, the atoms have specific coefficients 

 of attraction which vary with the nature of the atoms concerned.' The laws of 

 gravitation, with this proviso, were found to apply to all the definite cases examined, 

 and it was shown that a chemical change of combination is equivalent to a planetary 

 catastrophes So far the fundamental hypothesis of ' Newtonian Chemistry ' has led 

 to conclusions which are not at variance with the facts of the science, while it 

 gives promise of help in obtaining a solution of the great problem of the nature of 

 chemical action. 



Passing from considerations of the kind to which I have just referred, permit 

 me to occupy the rest of the time at my disposal with a short account of a line of 

 study in what I have already termed ' comparative chemistry,' which is not only of 

 inherent interest, but seems to give us the means of filling in some details of a 

 hitherto rather neglected chapter in the early chemical history of this earth. 



The most remarkable outcome of ' comparative chemistry ' is the periodic law- 

 of the elements, which asserts that the properties of the elements are connected in 

 the form of a periodic function wnth the masses of their atoms. Concurrently with 

 the recognition of this principle, other investigations have been in progress, aiming- 



