498 PBINCIPLES OF CHEMISTRY 



apparatus in tabular crystals, having a black grey colour and metallic 

 lustre.* 5 



The specific gravity of the crystals of iodine-is 4-95, It melts at 

 114 and. boils at 184. Its vapour is formed at a much lower tempera- 

 ture, and is of a violet colour, whence: iodine receives its name (toci&ys, 

 violet). The smell of iodine recalls the characteristic smell of hypo- 

 chlorous-acid ; it has a sharp sour taste. It destroys the skin and organs 

 of the body, and is therefore frequently employed for cauterising and as 

 an irritant for the skin. In small quantities it turns the skin brown, 

 but the coloration disappears after a certain time, partly owing to the. 

 volatility of the iodine. Water dissolves only g ^ part of iodine. A 

 brown solution is thus obtained, which bleaches, but much more feebly 

 than bromine and chlorine. Water which contains salts, and especially 

 iodides, in solution dissolves iodine in considerable quantities, and the 

 resultant solution is of a dark brown colour. Pure alcohol dissolves a 

 small amount of iodine, and in so doing acquires a brown colour, but 

 the solubility of iodine is considerably increased by the presence of a 

 small quantity of an iodine compound for instance, ethyl iodide in 

 the alcohol. 63 Ether dissolves a larger amount of iodine than alcohol 

 but iodine is particularly soluble in liquid hydrocarbons, in carbon bi- 

 sulphide, 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 quantities. 

 The blue coloration which free iodine gives with starch may also, 

 as has already been frequently- mentioned (see Chapter IV.), 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 



62 For the final purification of iodine, Stas dissolved it in a strong solution of 

 potassium iodide, and precipitated it by the addition of water (see Note 58). 



65 The solubility of iodine in solutions containing iodides, and compounds of iodine 

 In general, may serve, on the one hand, as an indication that solution is due to a similarity 

 between the solvent and dissolved substance, and, on the other hand, as an indirect proof 

 of that view as to solutions which was cited in Chapter I., because in many instances un- . 

 stable highly iodised compounds, resembling crystallo-hydrates, have been obtained from 

 such solutions. Thus iodide of tetramethylammonium, N(CH 5 ) 4 I, combines with I 2 and I 4 . 

 Even a solution of iodine in a saturated solution of potassium iodide presents indications 

 of the formation of a definite compound Klg. Thus, an alcoholic solution of KI 3 does 

 not give up iodine to carbon bisulphide, although this solvent takes up iodine from an 

 alcoholic solution of iodine itself (Girault t Jorgensen, and others). The instability of these 

 oompoirads resembles the instability of many crystallo-hydrates, for instance of HCl,2H a O. 



