856 PRINCIPLES OF CHEMISTRY 



and wax generally contain oxygen in addition to carbon and hydrogen, 

 but in relatively small proportion. There are also many hydrocarbons 

 which have the appearance of tar as, for instance, metacinnamene and 

 gutta-percha. Those liquid hydrocarbons which boil at a high tempera- 

 ture are like oils, and those which have a low boiling point resemble 

 ether, whilst the gaseous hydrocarbons in many of their properties are 

 akin to hydrogen. All this tends to show that in hydrocarbons physi- 

 cally considered the properties of solid non-volatile charcoal are 

 strongly modified and hidden, whilst those of the hydrogen predominate. 

 All hydrocarbons are neutral substances (neither basic nor acid), but 

 under certain conditions they enter into peculiar reactions. It has 

 been seen in those hydrogen compounds which have been already con- 

 sidered (water, nitric acid, ammonia) that the hydrogen in almost all 

 cases enters into reaction, being displaced by metals. The hydrogen of 

 the hydrocarbons, it may be said, has no metallic character that is to 

 say, it is not directly 22 displaced by metals, even by such as sodium and 

 potassium. On the application of more or less heat all hydrocarbons 

 decompose 23 forming charcoal and hydrogen. The majority of hydro- 

 carbons do not combine with the oxygen of the air or oxidise at ordi- 

 nary temperatures, but under the action of nitric acid and many other 

 oxidising substances most of them undergo oxidation, in which either 

 a portion of the hydrogen and carbon is separated, or the oxygen 

 enters into combination, or else the elements of hydrogen peroxide enter 

 into combination with the hydrocarbon. 24 When heated in air, hydro- 



with them ; the bare facts and some mechanical data exist, but their relation to molecular 

 mechanics has not been cleared up in the requisite degree. It has already been seen 

 from xisting data that the viscosity at the temperature of the absolute boiling point 

 becomes as small as in gases. 



w In a number of hydrocarbons and their derivatives such a substitution of metals 

 for the hydrogen may be attained by indirect means. The property shown by acetylene, 

 C 2 H 2 , and its analogues, of forming metallic derivatives is in this respect particularly 

 characteristic. Judging from the fact that carbon is an acid element (that is, gives an 

 acid anhydride with oxygen), though comparatively slightly acid (for carbonic acid is 

 not at all a strong acid and compounds of chlorine and carbon, even CC\4, are not decom- 

 posed by water as is the case with phosphorus chloride and even silicic chloride and boric, 

 chloride, although they correspond with acids of but little energy), one might expect to 

 find in the hydrogen of hydrocarbons this faculty for being substituted by metals. The 

 metallic compounds which correspond with hydrocarbons are known under the name of 

 organo-metallic compounds. .Such, for instance, is zinc ethyl, Z^CgHsJj, which corre- 

 sponds with ethyl hydride or ethane, C 2 H 6 , in which two atoms of hydrogen have been 

 exchanged for one of zinc. 



23 Gaseous and volatile hydrocarbons decompose when passed through a heated 

 tube. When hydrocarbons are decomposed by heating, the primary products are 

 generally other more stable hydrocarbons, among which are acetylene, C 2 H 2 , benzene, 

 CeH 6 , naphthalene", C 10 H 8 , <fcc. 



** Wagner (1888) showed that when unsaturated hydrocarbons are shaken with a 



