336 PRINCIPLES OF CHEMISTRY 



are wholly dependent upon the molecular weight and not upon the 'quality 

 of a substance, and that this gives the possibility of determining the 

 weight of molecules by studying these properties (for instance, the vapour 

 density, depression of the freezing point, &c.) It is apparent from the 

 foregoing that the physical and even more so the chemical properties of 

 homogeneous substances, more especially solid and liquid, do not depend 

 exclusively upon the weights of their molecules, but that many are in 

 definite (see Chapter XV.) dependence upon the weights of the atoms 

 of the elements entering into their composition, and are determined by 

 their quantitative and individual peculiarities. Thus the density of 

 Solids and liquids (as will afterwards be shown) is chiefly determined 

 by the weights of the atoms of the elements entering into their composi- 

 tion, inasmuch as dense elements (in a free state) and compounds are 

 only met with among substances containing elements with large atomic 

 weights, such as gold, platinum, and uranium. And these elements 

 themselves, in a free state, are the heaviest of all elements. Substances 

 containing such light elements as hydrogen, carbon, oxygen and nitrogen 

 (like many organic substances) never have a high specific gravity ; in 

 the majority of cases it scarcely exceeds that of water. The density 

 generally decreases with the increase of the amount of hydrogen, as the 

 lightest element, and a substance is often obtained lighter than water. 

 The refractive power of substances also entirely depends on the com- 

 position and the properties of the component elements. 29 bis The history 



29 bis With respect to the optical refractive power of substances, it must first be 

 observed that the coefficient of refraction is determined by two methods : (a) either all 

 the data are referred to one definite ray for instance, to the Fraunhofer (sodium) line 

 D of the solar spectrum that is, to a ray of definite wave length, and often to that red 

 ray (of the hydrogen spectrum) whose wave length is 656 millionths of a millimetre ; (6) 

 or Cauchy's formula is used, showing the relation between the coefficient of refraction and 



dispersion to the wave length n = A.+ , where A and B are two constants varying 



for every substance but constant for all rays of the spectrum, and A is the wave length 

 of that ray whose coefficient of refraction is n. In the latter method the investigation 

 usually concerns the magnitudes of A, which are independent of dispersion. We shall 

 afterwards cite the data, investigated by the first method, by which Gladstone, Landolt, 

 and others established the conception of the refraction equivalent. 



It has long been known that the coefficient of refraction n for a given substance 

 decreases with the density of a substance D, so that the magnitude (n 1) -r-D = C is 

 almost constant for a given ray (having a definite wave length) and for a given substance. 

 This constant is called the refractive energy, and its product with the atomic or mole- 

 cular weight of a substance the refraction equivalent. The coefficient of refraction of 

 oxygen is 1*00021, of hydrogen 1-00014, their densities (referred to water) are 0-00148 

 and 0*00009, and their atomic weights, O = 16, H = l ; hence their refraction equivalents 

 are 8 and 1*5. Water contains H 2 O, consequently the sum of the equivalents of 

 refraction is (2 x 1*5) + 8 = 6. But as the coefficient of refraction of water =* 1'881, 

 its refraction equivalent =5*958, or nearly 6. Comparison shows that, approxi- 

 mately, the sum of the refraction equivalents of the atoms forming compounds 



