Page:The New International Encyclopædia 1st ed. v. 04.djvu/654

* CHEMISTRY. 572 from the beginning the idea of radicles became associated with certain other ideas that could not be maintained in the light of more knowl- edge. Berzelius subdivided organic radicles, like the elements, into electropositive and electro- negative. On the other hand, it was generally expected that radicles would eventually be iso- lated and thus constitute a series of simple com- jHuiiids whose molecules would bear the same re- lation to the substances of organic diemistry as the atoms of the elements bear to the com- pounds of inorganic chemistry. But when the electrochemical theory was overthrown, while attempts to isolate radicles remained fruitless, The opinion began to spread that the theory of ladicles had made of organic chemistry a science of imaginary substances, and. hence, the sooner the theory was abolished the better for the yoimg science. Hut how, then, were organic compounds to be correlated? A solution of this problem was suggested by Diimas in 1S.'?!1. Continuing his researches on the substitution of dilferent elements for one another in organic compounds, Dumas found that in acetic acid hydrogen could be exchanged for chlorine, and that the resulting compound I trichloracetic acid) was very much like acetic acid itself. Similar facts had already been observed, since 1834. by himself as well as by Laurent. It now occurred to Dumas that in correlating their substances chemists could be guided solely by the phenomena of substitution. Acetic acid and its chlorine-substitution product obviously belong to the same 'type' and similar relations exist between other substances as well. If. therefore, the phenomena of substitution were investigated in connection with organic com- pounds in general, the result would be a group- ing of comi)ounds free from all hypothesis, but based on and exhibiting clearly their natural relationshi]). Such were, in mice. Dumas's views, on the basis of which the celebrated 'theory of types' was gradually built up in course of the fourth and fifth decades. The most important contributions to the theory were made by Ger- hardt, Wurtz, fTofmann. and Williamson. Ger- liardt realized that Dumas's ideas were worthy of being developed, but he also realized that this ciiuld not be done without the aid of the idea of radicles. Xo objection could be raised against the latter idea, once it were freed from all un- necessary associations, especially from the belief that radicles were unalterable substances capable of indeiiendent existence. To say that ben- zoyl chloride. C:II,.,0C1 ; benzoic acid, CjHcO.; and benzamide, C'lH^ON, contain in common the benzoyl radicle — i.e. the groip of atoms CrHjO — was only to express what was evident from their formulas. On the other hand, the recognition of radicles must obviously lead to the discovery of the relationship of compounds, and thus, to- gether with the |)heniimena of substitution, guide in grouping compounds in accordance with the id?a of types. In 1849 Wurtz and Hofmann discovered a series of compounds that bore an unmistakable resemblance to ordinarj' ammonia, and could be considered as ammonia in which one or nmrc hydrogen atoms were replaced by radicles. They proposed to group tlicm together as belonging to the 'ammonia type.' In 1850 Williamson showed that alcohols, ethers, and acids could be referred to the 'water type.' Or- ilinary alcohol, for instance, whose formula is C'.H,jO, could be considered as water, II.O, in CHEMISTRY. which one hydrogen atom has been replaced by the ethyl radicle. C.Hj. Ordinary ether, C,H,„0, could b(> considered as water. 11,. in which two hydrogen atoms have been replaced by two ethyl radicles, ether being thus (dHjl.O. Acetic acid. CiII.O,, could be considered as water, H.O, in which one hvdrogen atom has been replaced by the radicle ' aH:,0. Now. ether, (CjII,)jO, was obtained from alcoliol. C.ll.HO. by the use of dehydrating agents. Williamson therefore held, 1)y analogy, that it ought to be possible to transform acetic acid, C'~H,0.110, into a com- pound. (an,0):0. When, in 18.52. Frankland actually succeeded in ell'ccting this transforma- tion by the use of deliydrating agents, the use- fulness of the type theory was demonstrated. For nothing is more striking proof of the value of a theory than its capacity for revealing un- known facts. To the types ammoina and water Gerhardt added the tyjies hydrotirn and Inidro- chlurir acid, and for a time it seemed that all organic compounds could be giouped under these four simple types. It was soon found necessary, however, to introduce the idea.s of 'condensed types.' like the condensed water type, (HjO)., 'mixed types,' and the type mar.'.h (/uk. CH,. In course of the fifties the type theory thus gradu- ally became less and less simple, and, hence, less and less valuable for the purpose of correlating organic compounds. Meanwhile an idea of inestimable value had thrust itself U]ion chemists. Insjiecting the typ- ical formulas of compounds, they could not help noticing that certain radicles (e.g. methyl. CH,, or ethyl, C.H;.) were capable of replacing each a single atom of hydrogen ; others were cajjable of replacing each two atoms of hydrogen, etc. In other words, .some radicles were seen to be equivalent to an atom of hydrogen: others had double its com- bining capacity, etc. Hence the idea of the ralencji of radicles and atoms. Like most other general ideas, that of valency was not new. In a vague and more or less specialized form it may be found in the researches of Berzelius, Graham, Liebig, ,and others; and Frankland, who first clearly enunciated it, in 18,")2, justly points out that it was probably a vague recogniticm of the valency of radicles, as exhibited by the facts of substitution, that gave birth to the theory of types. Frankland's statements, however, at- tracted no attention. In 1858 Kekule and Couper independently developed the same idea, the latter proposing to symboliz* the combining capacity of different atoms liy the da.shes now generally employed in graphic formulas. Ke- kule called attention to the ipiadrivalcnce of the carbon atom, as shown dircctlv bv ccimpounds like the following: Cll,. CH,(T.' CII.CI.,. CHCl,, CCl,: or indirectly liy such compinmds as CO,. COCL. In the former compounds a single atom of carbon is seen to be equivalent to four atoms of hydrogen, and a single chlorine atom to a single atom of hydrogen, which is also shown by the fonnula of hydrochloric acid. HCl. In a compound like COCL, the o.xygen atom must therefore be assumed to be divalent, and so it is directly shown to be by the fornmla ILO. Ke- kule soon came to the conclusion thiit in practi- cally all organic comjiovmds one carbon atom is combined with a quantity of other elements which is equivalent to four atoms of hydrogen. Tliis gave rise to a lively controversy, tlic critic Kolbe especially maintaining that the valency of