SECT. i. NON-METALLIC SIMPLE SUBSTANCES. 21 



simple substances; lie considers them to be ozonides 

 analogous to the peroxides of manganese, lead, &c. He 

 believes chlorine to be the peroxide of murium, and bro- 

 mine to be the peroxide of bromium. Professor Tjndall's 

 experiments on the absorption and radiation of gases / 

 show that the action of these two substances is very^ 

 different from that of the simple gases. 



Iodine is a dark purple solid, crystallized in scales or 

 elongated octohedral plates. It slowly evaporates at 

 ordinary temperatures, and at that of 350 Fahr. it is 

 volatilized into a beautiful violet coloured gas which 

 changes starch into a bright blue, and for that reason a 

 little starch will detect the millionth of a grain of iodine 

 in composition. Iodine is slightly soluble in water, has 

 a hot acrid taste, and although used in medicine it is 

 poisonous when taken in large doses. Its bleaching 

 properties are inferior to those of its congeners, but its 

 chemical combinations are the same. With kydrogoii 

 it forms a highly explosive compound, which detonates 

 with the slightest pressure. 



These three simple substances are analogous in almost 

 every respect. They all possess a bleaching property, 

 many of their compounds are exceedingly explosive, 



-tuft hnrp in their gases, while 



their gases set fire spontaneously to substances generally 

 reckoned incombustible. Hence, though not combus- 

 tible, they support combustion, but in a very different 

 manner from oxygen. Chlorine and the gases of 

 bromine and iodine diluted with common air, do not 

 transmit blue and violet light ; that is to say, the spec- 

 trum of a sunbeam transmitted through them is deprived 

 of its most refrangible coloured rays, and that which 

 remains is crossed by more than a hundred equidistant 

 dark lines; their spectral properties however will be 

 given hereafter. They resemble oxygen in one respect 

 that when a current of electricity is passed continuously 



