CHEMISTRY. 



By George F. Barker, 

 Professor of Physics in the University of Pennsylvania, Philadelphia. 



GENERAL AND PHYSICAL. 



Williamson, in an address before the Chemical and Physical Society 

 of University College, London, discussed what he called an error in the 

 commonly received theory of chemistry. There is a division of opinion 

 on the question of variable equivalence; by one class of chemists nitro- 

 gen in ammonium chloride being considered quinquivalent, and by 

 another ammonium chloride being regarded as molecular: i. e., the 

 force uniting the compound together, according to the former, is atomic 

 or chemical; according to the latter, it is i)hysical. The author thought 

 we had no grounds for assuming a diflerence between chemical and 

 physical force. Kekule's theory, that an atom had only one valence, 

 was no longer tenable; nor, in his opinion, was the view that the valence 

 varied within narrow limits. He knew of no limitation to atomic value. 

 Contrary to what is often asserted, that the valence of an element was 

 independent of the nature of the elements with which it is combined, 

 we know that the character of these atoms materially affects the result. 

 Thus, gold, which afone could combine with no more than three chlorine 

 atoms, can take up an additional one if an atom of sodium be supplied 

 at the same time. The atomic value of an element depends upon the 

 nature of the combining atoms, and uijon the temperature also. {Nature, 

 IS'ovember, 1881, xxv, 21.) 



Perkin has obtained two series of compounds in his researches on 

 coumarine, differing in i^roperties, but generally convertible the one into 

 the other by the action of the heat. He thinks that the ordinary theorj' 

 of isomerism, according to which this phenomenon is traceable to the 

 occupation of different relative iiositions by the atoms in two molecules, 

 fails to exi)laiu the cases of isomerism now described by him. Jle favors 

 the view that the atoms in the molecules of any pair of the newly- 

 described compounds occupy the same relative positions, but are at 

 different absolute distances from each other. It should be remem- 

 bered, however, that the present theory of isomerism is a])plicable only 

 to gaseous molecules; the molecular phenomenon of liquid and solid 

 bodies are too complex to find as yet any general explanation. Perkiu's 



381 



