338 



NA TURE 



\Ang. 27, 1874 



the carbon is equally divided. The oxide is the same in both 

 acids, and is that oxide which occurs in oxalic acid. Tlie 

 hydride and the chloride have the composition of the substances, 

 the formuln; of which are Coll,; and CiClg respectively. Oxalic 

 acid undergoes chemical change much more readily than the 

 corTespondmg hydride or cliloride ; and therefore the chemical 

 character of acetic and of trichloracetic acids depends much 

 more on the oxidised than on the other constituent, and they 

 thus have a marked resemblance. The oxidised constituent is 

 united to the other in a manner different from that in which 

 oxalic acid is united to bases in the oxalates, inasmuch as, while 

 the basic water of hydrated oxalic acid is displaced when oxalic 

 acid unites with a base, in hydrated acetic and trichloracetic 

 acids there is the same proportion between the basic water and 

 the oxidised carbon as there is in oxalic acid. 



Now, has not this a great resemblance to the view entertained 

 by most modern chemists, that acetic acid is a compound of the 

 radical carboxyl (half a molecule of oxalic acid) and the radical 

 methyl (half a molecule of methyl gas), that trichloracetic acid 

 similarly contains the same radical carboxyl and the radical 

 C CI3, and that the prominent chemical properties of these 

 bodies depend upon their containing carboxyl, and that they 

 therefore resemble each other? 



The modern view contains nothing inconsistent with that of 

 Berzelius ; but it no doubt contains something more : it contains 

 an explanation of the difference between the manner in which 

 carboxyl is united to methyl in acetic acid, and the manner in 

 which oxalic acid is united to bases in the oxalates. But it will 

 surely be admitted that Berzelius was here far ahead of his oppo- 

 nents — so far ahead, that they altogether failed to see his mean- 

 ing, and looked upon his argument as a clumsy device. 



The treatment by Berzehus of the constitution of the sulpho- 

 acids, furnishes a precisely similar case. These are now regarded 

 as compounds of the radical .SO5OH (which we may call sulph- 

 oxyl). This radical is half a molecule of hyposulphuric acid ; 

 and Berzelius considered them coupled compounds of hyposul- 

 phuric acid, adopting at once the view first brought forward by 

 Kolbe in his classical memoir on the sulphite of perchloride of 

 carbon and the acitls derived from it. 



I might pursue the history of the carbon- and sulpho-acids 

 further, and trace the development of the theory of their consti- 

 tution through the discoveries of Kolbe, and his beautiful appli- 

 cation to the cases of carbon and sulphur of Frankland's lai- 

 sighted speculation on the constitution of the organo-metallic 

 bodies, pointing out the relation of Kolbe's views of the consti- 

 tution of acids, alcohols, aldehydes, and ketones, to the Berze- 

 lian theory on the one hand, and to the opinions of modern 

 chemists on the other ; but the greater part of such an historical 

 sketch has been given very recently by Kolbe himself in the 

 yoiiDial fiir p/-iikfisi/u' Cheni'u\ and 1 may therefore omit it. 



It would be easy to bring forward cases to show that our 

 present views can be directly derived from the substitution theory 

 and_ the types of Dumas and Gerhardt, through the complica- 

 tions of multiple and mixed types, and the labyrinthine formulre 

 to which these gave rise, to the wonderfully simple and compre- 

 hensive system of Kekule ; but that is unnecessary, as this 

 development has been fully and ably described by more than one 

 thoroughly competent %vriter. 



We have been discussing a case in which Berzelius was right 

 in considering a compound of carbon, oxygen, and chlorine as 

 composed of two parts — an oxide and a chloride of carbon. It 

 is only just that we should only take some notice of cases, at 

 first sight similar, in which modem chemists would be inclined 

 to think that he was wrong. This is the more necessaiy, as an 

 examination of these cases will enable us to see what was the 

 really valuable contribution made to speculative chemistry by the 

 substitution theory. 



Compounds containing three elements were formulated in two 

 different ways by Berzelius : — ■ 



1. One of the elements was represented as combined with 

 a radical composed of the other two, as — hydrocyanic acid, 

 H2-C.>N„ ; ether, C.,lIi„-0. 



2. fhe ternary compound was represented as composed of 

 two binary compounds, having one element common, as — caustic 

 potash, KO,H„0 ; chromochloric acid, 2CrO;„ CrCl,,. 



Phosgene gas was at first formulated in the former of these 

 ways as CO, CI., ; but latterly he was forced, in consistency, to 

 give up all radicals containing oxygen or other strongly electro- 

 negative element,* and to write the formula of phosgene gas 



* In 183S Herzeliii5 was incfined to regard C.O-., In wliich fie gave llie 

 name "oxatyf," as tlic radical of n\alic acid and oxamidc. 



CO„, CCI4. Similarly, in every case where a positive element 

 or radical is combined with two negative elements or radicals, he 

 represented the compound as composed of two binary com- 

 pounds, thus — chloride of acetyl, 2CjH„03, C4H,jClg, as a com- 

 pound of acetic acid and the corresponding terchforide. 



This was in perfect consistency with the mode in which ternary 

 compounds containing one negative and two positive elements or 

 radicals \rere formulated, as caustic potash, KO, H„0, sulphate 

 of copper, CuO, SO;,, &c. ; but it lacks the practical justifica- 

 tion which can be given for the formula C.IIp, C2O3 for acetic 

 acid ; for phosgene acts readily on water, forming carbonic and 

 hydrochloric acids, an action which does not take place with 

 perchloride of carbon ; and it is not easy to see why the latter 

 substance should be more readily attacked by water when com- 

 bined with carbonic acid than when free. This difference did 

 not escape the attention of Berzelius, and led him to distinguish 

 two modes of chemical union : (i) where the constituents were 

 held together by the electro-chemical force, and wholly or par- 

 tially neutralised each other, as in the oxygen and sulphur salts ; 

 and (2) where a so-called ** copula " was attached by an unknown 

 force to a substance Iwithouc greatly modifying its chemical 

 activity. The distinction seems arbitrary ; but it was not, as is 

 usually supposed, a mere artificial bulwark to protect the electro- 

 chemical theory ; it has a real and very important meaning, a 

 meaning which the development of the substitution theory enables 

 us to explain. 



The phenomena of electrolysis, upon which the Berzelian 

 system is based, bring forward into great prominence one of the 

 chemical units, viz. the equivalent ; and the pre-eminent position 

 of oxygen as the most electro-negative element made it most 

 natural to select the atom of oxygen as the standard of equiva- 

 lence, so that an equivalent of any element or radical was defined 

 as that quantity of it which is equivalent to one atom of oxygen. 

 Gay-Lussac's law of gaseous volumes, which was adopted by 

 Berzelius, and which, by a curious accident, happens to be true 

 for all elements gaseous at ordinary temperatures, led to the for- 

 mula* H., and Cl„ for the cquivalcitls of hydrogen and chlorine ; 

 but although these formula explicitly indicate the divisibility of 

 the equivalents of these elements, this divisibility was not recog- 

 nised, and integral numbers of equivalents were alone tolerated. 

 Thus hydrochloric acid was written H^C!;, ammonia NoH^, c\:c. , 

 and the etymological meaning of the word atom was soon lost. 

 The use ol barred letters to indicate two atoms or one equivalent 

 of such elements as hydrogen and chlorine further contributed to 

 hide the important fact of their divisibUity. 



The first great result of the substitution theory was to change 

 the unit of equivalence, and to take as the standard the atom of 

 hydrogen or of chlorine instead of that of oxygen ; and although 

 it would be most unjust to forget the services of Dumas, Gerhardt, 

 Laurent, and Odling in this matter, the credit of removing the 

 bars from H, CI, and their comrades, and allowing the hitherto 

 chained partners to walk at liberty, undoubtedly belongs mainly 

 to our distinguished colleague and master, Trof. Williamson. 



The establishment of the water type, or (to put it in another 

 form) the proof that the atom of oxygen contains two units of 

 oxygen, inseparably united but capable of separate action, led 

 the way to the explanation of all the difficulties which beset the 

 theory of radicals and copula;. It at once explained how tv.'o 

 oxides or two sulphides unite together ; * and the idea of "poly- 

 basic," or, as we should now say, polyad atoms and radicals, 

 was soon used to explain the existence of polyb.isic acids, double 

 salts, acichlorides, and many other kinds of ternary compounds. 



But a fact does not cease to exist because it is explained. 

 Quicklime and water unite together, although we can now explain 

 how they do so ; and a useful purpose may still be served by the 

 enumeration, as in the old dualistic formuUx", of the pairs of 

 united equivalents. Although some of these etiuivalents belong 

 to the same atoms, it is nevertheless true that they are united in 

 pairs. Caustic potash might thus be formulated, KOJ, HO.', or 

 .^(KjO.HjiO); phosgene gas, .l(COo, CCI4); and chlorochromic 

 acid, !|(2Cr03, CrCl,;). These fonnuki; are not so well suited 

 for general use as those now current ; but the consideration of 

 them as accurate representations of facts may enable us to see 

 that the copula; of Berzelius had a real and valuable meaning. 

 Take, for instance, the formula of acetic acid, II3C — CO— OH, 

 or jCHj, i|'CO._., iH.,0,4C2 ; it is this last term which indicates 

 the coupled character of the compound. If we look upon acetic 

 acid as a compound of carbon, it is a coupled compound because 



* It does not explain the existence of double clilorldes, bromides, &c. 

 These compounds, apparently so similar to the double oxides and sulphides, 

 are still unexplained. 



