342 Prof. Draper on the Allotropism of Chlorine^ 



of a transient kind, but remaining for a length of time, we see 

 how it is that after an exposure to the sun decomposition is 

 subsequently carried forward in the dark. 



If the power which chlorine assumes, of uniting with hy- 

 drogen and carbon, depends on a change in its electrical re- 

 lations, a passage from the passive to the active state, we might 

 expect that those various causes, which in the case of other 

 elementary bodies bring about analogous changes, and throw 

 them from one allotropic condition to another, would here also 

 exercise a perceptible action. Among such causes we may 

 enumerate the action of a high temperature, and the contact 

 or presence of other bodies. 



It may be remarked, in the instances to which Berzelius 

 has referred, that exposure to a high temperature is one of 

 the most frequent causes of allotropic change. In the case of 

 chlorine the remark holds good, for, as is well known, when 

 a mixture of chlorine and hydrogen is passed through a red- 

 hot tube, chlorohydric acid forms with rapidity. The high 

 temperature therefore impresses upon chlorine the same ten- 

 dency to unite with hydrogen which is communicated by the 

 solar rays. 



But the contact of other bodies frequently determines in a 

 given substance an allotropic change. Thus, when a piece of 

 iron is placed in nitric acid in contact with platina, the iron 

 becomes less electro-positive, or, what is the same thing, more 

 electro-negative than it was before, and the acid can no longer 

 oxidize it. The contact of the very same substance, platina, 

 determines an analogous change in chlorine, giving it at once 

 the capacity of uniting with hydrogen. The porous condition 

 of spongy platina is not essential to the result, for clean pla- 

 tina-foil exhibits the same phaenomenon. 



In the case of iron, the action of a high temperature on the 

 contact of platina throws the metal from the active to the pas- 

 sive state ; in the case of chlorine, the same causes apparently 

 produce the opposite result, throwing the gas from the passive 

 to the active state. But the difference is rather in appearance 

 than in reality. In both cases it amounts to the same thing, 

 and is an exaltation of the electro-negative qualities of either 

 substance respectively. 



The same causes therefore which produce allotropic changes 

 in other bodies produce analogous changes in chlorine. 



Now among the physical facts connected with the theory of 

 types and substitutions, two are prominent: — 1st, the union of 

 chlorine with hydrogen, giving rise to the removal of that hy- 

 drogen as chlorohydric acid ; 2nd, the subsequent function 

 discharged by the chlorine which has entered as an integrant 



