Mi M..H: XX. J THE ALLOTROl'ISM OF CHLOHINK. 



808 



flame. As is well known, there are two forms of silicic 

 MrM: one soluble in water and hydrochloric acid, but 

 passing into the insoluble state by being previously 

 made red-hot. The silicon therefore carries in its com- 

 bination the same properties that it exhibits in the free 

 state. 



In the same manner it might be shown that sulphur, 

 selenium, phosphorus, titanium, chromium, uranium, tin, 

 iridium, osmium, copper, nickel, cobalt, and a variety of 

 other bodies, exist under several different forms, with 

 distinctive properties that are often well marked. In 

 several of them the influence of this allotropic condition 

 is plainly carried into the compounds, as is well shown 

 in the two varieties of arsenic which give rise to the two 

 arsenious acids. 



The passage from one allotropic state to another takes 

 place commonly through the agency of apparently very 

 trivial causes, such as a slight elevation of temperature 

 and the contact of certain bodies. Thus iron, which is 

 so easily oxidized under ordinary circumstances, appears 

 to lose its affinity for oxygen after it has been touched 

 under the surface of nitric acid by a piece of platinum. 

 It then puts on the attributes of a noble metal, and sim- 

 ulates the properties of platinum and gold. 



This remarkable instance of the passage from an act- 

 ive to a passive state, as Berzelius remarks, may lead to 

 a conjecture respecting the true condition of certain 

 gases. No one can reflect on the inactivity of nitrogen 

 gas under ordinary circumstances, contrasted with its 

 equally extraordinary activity as a constituent of organ- 

 ic bodies, without being struck with the apparent con- 

 nection of that phenomenon with these of allotropisui. 

 And though Berzelius with his customary caution mere- 

 ly insinuates that nitrogen can exist under two forms, 

 the facts here developed in relation to chlorine appear to 



