324 PRINCIPLES OF CHEMISTRY 



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

 weights of the atoms of the elements entering into their composition, 

 inasmuch as heavy (free) elements and compounds are only met with 

 among substances containing elements with large atomic weights, sucli 

 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 augmentation 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. 30 The history 



individual functions of the cells. The further development of the questions touching 

 on this subject should in this manner not only aid the perfecting of the theory of solu- 

 tions but also the further progress of physiological science. 



50 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 Frauenhofer (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 million parts of a millimetre ; (b) or 

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



T> 



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

 ing 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. 



The coefficient of refraction n for a given substance decreases, as has long been 

 known, with the density of a substance D, so that the magnitude (n l) -=-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 molecular 

 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 0'00143 and 0'00009, 

 and their atomic weights, O = 16, H = l ; hence their refraction equivalents are 3 and 1-5. 

 Water contains H 2 O, consequently the sum of the equivalents of refraction is (2 x 1'5) + 

 3 = 6. But as the coefficient of refraction of water = 1-331, therefore its refraction equi- 

 valent = 5"958, or nearly 6. The comparison shows that, approximately, the sum of the 

 refraction equivalents of the atoms forming compounds (or mixtures) is equal to the re- 

 fraction equivalent of the compound. According to the researches of Gladstone, Landolt, 

 Hagen, Briihl and others, the refraction equivalent of the elements are H = l'8,Li = 8'8, 

 B = 4'0, C = 5'0, N = 4-l (in its highest state of oxidation, 5'3), = 8*0, F = T4, Na = 4'8, 

 Mg = 7-0, Al = 8-4, Si = 6'8, P - 18'8, S = 16'0, Cl = 9'9, K = 8*1, Ca = 10'4, Mn - 12'2, Fe = 12'0, 

 (in the salts of its higher oxides 20'1), Co = 10'8, Cu = ll'6, Zn = 10'2, As = 15'4, Bi = l.V:',, 

 Ag = 15'7, Cd = 13-6, 1 = 24-5, Pt = 26'0, Hg = 20'2, Pb = 24'8, &c. The refraction equi- 

 valents 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 appl i<- 1. 

 Nevertheless the conception of the refraction equivalents gives an easy means for directly, 



