LECTURE X.] HISTORY OF CHEMISTRY. 189 



equivalents of carbon, oxygen, and sulphur are not 6, 8, and 

 1 6, as Gmelin's school assumed, but twice these numbers, that 

 is, 12, 16, and 32; and he proves, by many examples, that 

 there do not exist any equivalent formulae, constructed on the 

 principles advanced by himself, which contain less of the re- 

 spective elements than these latter quantities. He also proves 

 that an even number of atoms of carbon, of oxygen, and of 

 sulphur always occurs in compounds containing these elements, 

 if they are represented by means of Gmelin's equivalents. 



Consequently Gerhardt doubles the equivalents of carbon, 

 of oxygen, of sulphur, etc., with reference to those of hydrogen, 

 of chlorine, of nitrogen, etc., whereby he obtains Berzelius' 

 numbers. He differs very materially from the followers of 

 Berzelius in the formulae which he proposes for organic com- 

 pounds. According to him, these had been doubled, as com- 

 pared with many inorganic substances, consequently he halves 

 them ; they were, as he expresses it, referred to H = 2, or to 

 = 200, whilst, for the majority of inorganic compounds, the 

 number chosen for compariso'n was H= i or O= 100. Hence 

 substances were divided into those like water, carbonic oxide, 

 carbonic acid, etc., which occupy two volumes (H = i = i vol.), 

 and those like alcohol, ethylene, chloride of ethyl, etc. (that is, 

 all the substances which were then called organic, and of which 

 the vapour densities were by no means always known), which 

 correspond to double this volume. 



I may be permitted to leave off the consideration of Ger- 

 hardt's views here, in order to glance backwards and seek for 

 the reasons which had caused chemists to write "four-volume" 

 formulae for organic substances. This way of writing them 

 must appear all the more remarkable since both Berzelius 

 and Dumas, at first, at least, believed that they must choose the 

 atomic weights of compounds so that they should represent 

 equal volumes in the state of vapour. 



Relatively few vapour densities were known at that time, 

 and hence the rule was broken in many cases without the fact 

 being known. Another very important reason lay in the widely 

 spread assumption that the acid was the substance united to 



