410 



SCIENTIFIC THOUGHT. 



found that one or more atoms in an organic compound, 

 notably of hydrogen, might be replaced by an equal 

 number of atoms of other elements, and that such pro- 

 ducts of substitution retained similar qualities, and could 

 be mutually converted into each other, the type of the 

 compound remaining the same. The process of substitu- 

 tion led to the conception of " Types," which remained 

 the same whilst the individual compounds varied ac- 

 cording to the different elements which were introduced. 



gen acids of chlorine, bromine, and 

 iodine, and the investigations of 

 Graham into the salts of phosphoric 

 acid and its different modifications. 

 Davy, though together with Ber- 

 zelius the founder of the electro- 

 chemical theory, had found it neces- 

 sary to modify the oxygen theory 

 of Lavoisier viz., that oxygen was 

 necessarily the acid - forming ele- 

 ment : he, and after him Dulong 

 in France, had explained the so- 

 called oxygen acids like sulphuric 

 acid as hydrogen compounds of 

 certain compound radicles (S0 4 ) 

 exactly as hydrochloric acid is a 

 hydrogen acid of the simple radicle 

 chlorine. Graham's discovery of 

 three modifications of phosphoric 

 acid, and of the different power of 

 saturation of these three modifica- 

 tions, led to long discussions as to 

 what is really meant by a neutral 

 salt. Liebig in the year 1838, in 

 an important memoir gathering to- 

 gether the conclusions which these 

 facts, not easily reconciled with 

 Berzelius's system, had led him to, 

 emphasised there the twofold pos- 

 sibility of regarding metallic salts 

 either with Berzelius as binary 

 combinations of oxides with an- 

 hydrous acids, or else as products 

 of substitution of hydrogen com- 

 pounds, hydrogen being replaced 

 by metals. The choice might then 



depend on considerations of con- 

 venience : the one view might be 

 more suitable for inorganic notably 

 metallic compounds, the other for 

 organic compounds. The hydrogen 

 theory was thus introduced along- 

 side of the oxygen theory ; substi- 

 tution was introduced alongside of 

 simple combination. Though in 

 this stage the radicle theory was 

 already threatened, it was still pos- 

 sible to uphold the binary theory, 

 though it was not necessary. Chlo- 

 rine could act in the same way as 

 oxygen, being an electro-negative 

 element. But when, in pursuing 

 the line of investigation opened out, 

 it was found that chlorine, the 

 electro-negative element, could take 

 the place of hydrogen in organic 

 compounds without changing their 

 chemical character, the binary 

 theory, based upon polar (electri- 

 cal) contrasts, became insufficient 

 as a means of explanation or even 

 of classification. Dumas was the 

 first to indicate this (1834), though 

 he attempted to save the electro- 

 chemical or polar theory by stating 

 that the two electrically opposite 

 constituents of an organic com- 

 pound might contain the same 

 elements in the opposite electrical 

 positions (Kopp, ' Entwickelung der 

 Chemie,' pp. 564, 595, &c.) 



