"MOLECULES AND ATOMS 837' 



of chemistry presents a striking example in point Newton foresaw 

 from the high refractive index of the diamond that it would contain 

 a combustible substance since so many combustible oils have a high 

 refractive power. We shall afterwards see (Chapter XV.) that 

 many of those properties of substances which are in direct dependence 

 not upon the weight of the molecules^ but upon their composition, or, in 

 other words, upon the properties and quantities of the elements enter- 

 ing into them, stand in a peculiar (periodic) dependence upon the 

 atomic weight of the elements ; that is, the mass (of molecules and 

 atoms), proportional to the weight, determines the properties of 

 substances as it also determines (with the distance) the motions of the 

 heavenly bodies. 



(or mixtures) is equal to the refraction equivalent of .the compound. According to the i 

 researches of Gladstone, Landolt, Hagen, Briihl and others, the refraction equivalents of 

 the elements are H = 1'8, Li = 3.8, B = 4'0, C = 5'0, N = 4'l (in its highest state of oxida- 

 tion, 5-3), O = 2-9, F = 1'4, Na = 4'8, Mg = 7'0, Al = 8'4, Si = 6'8, P = 18'3, S = )6'0, Cl = 99, 

 K = 8-l, Ca = 10'4, Mn = 12-2, Fe = 12'0 (in the salts of its higher oxides, 20'1), Co = 10'8, 

 Cu = H-6, Zn = 10% As = 15-4, Bi = 15'3, Ag = 15'7, Cd = 13'6, I = 24 : 5, Pt = 26'0, Hg = 20-2,| 

 Pb = 24*8, &c. The refraction equivalents of -many elements could only be calculated) 

 from the solutions of their compounds. The composition of a solution being known it is 

 possible to calculate the refraction, equivalent of one of its component parts, those for all 

 its other components being known. The results are founded on the acceptance of a law 

 which cannot be strictly applied. Nevertheless the representation of the refraction 

 equivalents gives an easy means for directly, although only approximately, obtaining the 

 coefficient of refraction from the chemical composition of a substance. F6r instance, 

 the composition of carbon bisulphide is CS.i= 76, and from its density, 1'27, we find its 

 coefficient of refraction to be T618 (because the refraction equivalent =5 + 2x16 = 87), 

 which is very near the actual figure. It is evident that in the above representation com- 

 pounds are looked on as simple mixtures of atoms, and the physical properties of a com- 

 pound as the sum of the properties present in the elementary atoms forming it. If this 

 representation of the presence of simple atoms in compounds had not existed, the idea 

 of combining by a few figures a whole mass of data relating to the coefficient of refrac- 

 tion of different substances could hardly have arisen. For further details on this subject, 

 see works on Physical Cfomistry, 



