718 DR A. CRUM BROWN ON ISOMERIC COMPOUNDS. 



there are four, and in pyrotartaric acid six, other atoms of hydrogen present. 

 This hydrogen is considered, according to the typical theory, to belong to the radi- 

 cal, according to the theory of atomicity, to be directly united to the carbon, and 

 in such a way that there are always two atoms of hydrogen united to the same 

 carbon atom. If we noAv suppose, that in the one or the other of these two nor- 

 mal acids two such hydrogen atoms are wanting, we have, on the one hand, the 

 composition of fumaric and maleic acids, on the other, the formula of citraconic 

 itaconic, and mesaconic acids. Now, as there are in succinic acid two pairs 

 of hydrogen united in this Avay to carbon, we easily see the possibility of the 

 existence of two dehydrogenated acids, as the one or the other of these pairs of 

 hydrogen atoms is absent. 



" Similarly, in the case of pyrotartaric acid, the existence of three isomeric 

 dehydrogenated acids is intelligible, in each of which another of the three pairs of 

 hydrogen atoms directly united to carbon is absent. At that place in the mole- 

 cule where the two hydrogen atoms are wanting, there are two carbon affinities 

 unsaturated, there is at that place, so to speak, a blank."* 



There is some difficulty in understanding this last statement. For what can 

 be meant by two affinities of the same carbon atom uniting together ? unless 

 the definition of either " atoms" or " combination" be completely changed. Or 

 if we take the natural meaning of the sentence last quoted, and suppose two car- 

 bon atoms pushed together, so that two affinities of each previously united to 

 hydrogen come to be united together, the two wanting hydrogen atoms do not 

 come from the same, but from two different carbon atoms. f 



But leaving this preliminary difficulty out of consideration, and also granting 

 the existence in such a molecule of diatomic carbon, a glance at the diagrams is 

 sufficient to show, that this vicAv does not give us two but only one formula for 

 fumaric and maleic acids, unless a difference be admitted among the affinities of 

 carbon. The pairs of hydrogen atoms, which I have marked a and h, have per- 

 fectly similar positions, the one being related to one end of the diagram, exactly 

 as the other is to the other end. In the same way the graphic representation of 

 pyrotartaric acid suggests only two formulae for citraconic, itaconic, and mesa- 

 conic acids, there being no apparent difference between the position of the pairs 

 c and e.\ This explanation is insufficient, not to say unintelligible, unless we 

 suppose a functional difference between the two carbon atoms, with which the 

 pairs of atoms of hydrogen are united ; and if we make this assumption, it fol- 

 lows as a necessary consequence, that there is a difference between the two 

 groups of HO. It is no doubt possible, that such a difference may exist; but it 



* " It may of course equally well be assumed, that the carbon atoms are, as it were, pushed 

 together (zusammengeschoben), so that two carbon atoms are united by two affinities of each. This 

 is only another form of the same idea." 



f See Eklenmeyer, Zeltschrift, vi. 21. * % See Butlerow, Zeitschrift, vi. 524. 



