340 Prof. Draper on the Allotropism of Chlorine, 



are isomeric bodies, may perhaps consist in this, — that in the 

 former the carbon is under the form of common charcoal, and 

 in the latter under the form of diamond. 



The following instances from Berzelius may serve as exam^ 

 pies of these allotropic states. -' ' 



Carbon is known under three forms, charcoal, plumbago, 

 and diamond ; they differ in specific gravity, in specific heat, 

 and in their conducting power as respects caloric and electri- 

 city. In their relations to light the one perfectly absorbs it, 

 the second reflects it like a metal, the third transmits it like 

 glass. In their relations with oxygen they also differ surpri- 

 singly ; there are varieties of charcoal that spontaneously take 

 fire in the air, but the diamond can only be burnt with diffi- 

 culty at a high temperature in pure oxygen gas. The second 

 and third varieties do not belong to the same crystalline form. 



Silicium exists also under two forms. In its first it burns 

 with facility in the air under a slight elevation of temperature ; 

 but if it be previously exposed to a strong red heat, it changes 

 into the second variety and becomes incombustible, so that it 

 v/ill not oxidize when placed with nitrate of potash in the hot- 

 test part of the blowpipe flame. As is well known, there are 

 two forms of silicic acid, one soluble in water and hydrochloric 

 acid, but passing into the insoluble state by being previously 

 made red-hot. The silicium, therefore, carries in its combi- 

 nations the same properties that it exhibits in the free state.^^ ' 



In the same manner it might be shown that sulphur, sele- 

 nium, phosphorus, titanium, chromium, uranium, tin, iridium, 

 osmium, copper, nickel, cobalt, and a variety of other bodies 

 exist under several different forms, with distinctive proper- 

 ties that are often well-marked. In several of them the influ- 

 ence of this allotropic condition is plainly carried into the com- 

 pounds, 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 slight elevation of temperature, and the contact of cer- 

 tain bodies. Thus iron, which is so easily oxidized under or- 

 dinary circumstances, appears to lose its affinity for oxygen 

 after it has been touched under the surface of nitric acid by a 

 piece of platina. It then puts on the attributes of a noble 

 metal and simulates the properties of platina and gold. 



This remarkable instance of the passage from an active 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 cir- 

 cumstances, contrasted with its equally extraordinary activity 



