376 THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII. 



sent analogous results. Silicon, sulphur, selenium, phos- 

 phorus, titanium, chromium, uranium, tin, iridium, osmi- 

 um, copper, nickel, cobalt, iron, oxygen, chlorine, are cases 

 in point; and the instances which appear as exceptions 

 are rapidly diminishing in number. 



As is well known, to these singular modifications 

 Berzelius gave the designation of allotropic forms, and 

 the whole phenomenon passes conveniently under the 

 designation of allotropism. He shows that the peculiar- 

 ity assumed is often of such a persistent nature that it is 

 not lost, even though the substance affected should go 

 into combination with others. Thus, there are two forms 

 of silicon one combustible, and the other remarkably 

 incombustible. Each, by uniting with oxygen, gives rise 

 to silicic acid ; the acid in one case being soluble in 

 water and in hydrochloric acid, and in the other the re- 

 verse. And, in like manner, metallic arsenic, which ex- 

 hibits the same duality of condition, gives rise to two 

 different arsenious acids. Of phosphorus there are at 

 least two modifications ; and, accordingly, we have two 

 compounds of that body with hydrogen, one of which is 

 spontaneously inflammable, and the other not ; and at 

 least two oxygen acids, the monobasic and tribasic, in 

 which the essential difference rests in the state of the 

 phosphorus they contain. 



It is to be remarked that, so far as observation ex- 

 tends, the most common cause of producing these singu- 

 lar differences is the action of that class of agents which 

 we term imponderable substances. In very many cases, 

 change of temperature brings about allotropic change; 

 in others it is the agency of light, as in chlorine and 

 phosphorus ; and again, in others, association with for- 

 eign bodies, which apparently establish new voltaic rela- 

 tions. Heat, light, and electricity seem to be the general 

 modifying agents. 



