492 PRINCIPLES OF CHEMISTRY 



iodine is particularly soluble in liquid hydrocarbons, in carbon bisulphide., 

 and in chloroform. A small quantity of iodine dissolved in carbon 

 bisulphide tints it rose-colour, but in a somewhat larger amount it 

 gives a violet colour. Chloroform (quite free from alcohol) is also 

 tinted rose colour by a small amount of iodine. This gives an 

 easy means for detecting the presence of free iodine in small quan- 

 tities. The coloration which free iodine gives with starch may 

 also, as has already been frequently mentioned, serve for the detection 

 of iodine. 



If we compare the four elements, fluorine, chlorine, bromine, and 

 iodine, we see in them an example of analogous substances which, 

 arrange themselves by their physical properties in the same order as 

 they stand in respect to their atomic and molecular weights. If the- 

 weight of the molecule be large, the substance has a higher specific- 

 gravity, a higher melting and boiling point, and a whole series of pro- 

 perties depending on this "difference in its fundamental properties. A 

 large atomic weight should determine a larger affinity between the- 

 molecules, and consequently a greater difficulty in their division and a 

 greater attraction between them. We meet a very evident example- 

 in question in polymeric compounds ; for instance, in the hydrocarbons- 

 expressed by the formula, C rt H 2B : C 2 H 4 , C 3 H G are gases, those having a 

 greater molecular weight for instance, C 5 H 10 , C 7 H 14 , &c. areliquids, 

 and those of a still greater molecular weight are solids. The same rela- 

 tion is found in the above-named four elements. Chlorine in a free 

 state boils at about 35, bromine boils at 60, and iodine only above 

 180. According to Avogadro-Gerhardt's law, the vapour densities 

 of these elements in a gaseous state are proportional to their atomic 

 weights, and here, at all events approximately, the densities in a liquid 

 (solid) state are also almost in the ratio of their atomic weights. 

 Dividing the atomic weight of chlorine (35-5) by its sp. gr. in a liquid 

 state (1'3), we obtain a volume = 27, for bromine (80/3'1) 26, as also 

 for iodine (127/4-9) 26.' J1 



The metallic bromides and iodides are in the majority of cases and 



64 The equality of the atomic volumes of the halogens themselves is all the more 

 remarkable because in all the halogen compounds the volume augments with the substitu- 

 tion of fluorine by chlorine, bromine, and iodine. Thus, for example, the volume of 

 sodium fluoride (obtained by dividing the weight expressed by its formula by its specific- 

 gravity) is about 15, of sodium chloride 27, of sodium bromide 82, and of sodium 

 iodide 41. The volume of silicon chloroform, SiHCl,-, and of the corresponding bromine, 

 and iodine compounds, are 82, 108, and 122 respectively. So in solutions, for example, 

 NaCl + 200H 2 O has a sp. gr. (at 15/4) of 1'OIOG, consequently the volume of the solution 

 8658-5/1-0106 = 8620, hence the volume of sodium chloride in solution =3620-8603 (this is 

 the vol. of 200 H 2 O) = 17. So also in similar solutions, NaBr = 26 and Nal = 85. 



