442 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



COMPOUNDS, which " do not form vapours, but are decom- 

 posed by the action of heat, reproducing the molecules 

 which gave birth to them " (ibid. 513). Thus he wrote 



NH 3 + HC1 = NH 3 ,HC1 not NH 4 C1 

 PC1 3 + C1 2 =PC1 3 ,C1 2 ^PC1 5 



Frankland, on the other hand, in his first note on the 

 combining-power of the elements (1852) had already 

 adopted the idea of variable valency by ascribing to 

 nitrogen and phosphorus a tendency to unite with three or 

 five equivalents of other elements. This idea, in one form 

 or another, is now adopted very widely. Odling's attempt 

 to distinguish between " artiads " of even valency and " peris- 

 sads " of uneven valency (Phil. Mag., 1864, 27, 115-119) 

 broke down when it was found to lead to purely fictitious 

 formulae, such as N 2 O. 2 for nitric oxide, NO, and to unjusti- 

 fied contrasts such as those shown by the formulae 

 FeCl 2 ,Fe 2 Cl fi (for FeCl 3 ) and KQO 4 ,K 2 M n;J O 8 (for KMnO 4 ). 



SUMMARY AND SUPPLEMENT. 

 A. THE RISE OF ORGANIC CHEMISTRY. 



The rapid growth of inorganic chemistry between 1766 and 

 1816 is paralleled by the rise and development of organic 

 chemistry in the fifty years from 1815 (when Berzelius first 

 assigned formulas to organic compounds) to 1865 (when Kekule 

 put forward his structural formula for benzene). The recogni- 

 tion of organic substances as compounds of carbon and hydrogen 

 with oxygen, and sometimes nitrogen and phosphorus, is due to 

 Lavoisier (1784-1789), who also made the first organic analyses. 

 He estimated the carbon and hydrogen in charcoal, olive-oil, 

 wax and spirit of wine (1784) and showed (1789) that in vinous 

 fermentation there is a balance between the carbon, hydrogen, 

 oxygen, and nitrogen in the materials used (sugar, water, yeast) 

 and in the products obtained (alcohol, carbonic anhydride, etc.). 

 In doing this he enunciated the principle of the CONSERVATION 

 OF MASS in chemical changes and explained the significance of 



