370 PRINCIPLES OF CHEMISTRY 



one will have the ethylene structure, CH 2 XCH 2 X, and the other an 

 ethylidene structure, CH 3 CHX 2 ; such are, for instance, ethylene 

 chloride, CH 2 C1CH 2 C1, and ethylidene chloride, CH 3 CHC1 2 . And as 

 in the place of the first atom of hydrogen not only metals may be substi. 

 tuted, but Cl, Br, T, OH (the water radicle), NH 2 (the ammonia radi- 

 cle), N0 2 (the radicle of nitric acid), <fec., so also in exchange for two 

 atoms of hydrogen 0, NH, S, &c., may be substituted ; hence it will 

 be understood that the quantity of isomerides is sometimes very great. 

 It is impossible here to describe how the isomerides are distinguished 

 from each other, in what reactions they occur, how and when one 

 changes into another, &c. ; for this, taken together with the descrip- 

 tion of the hydrocarbons already known, and their derivatives, forms a 

 very extensive and very thoroughly investigated branch of chemistry, 

 called organic chemistry. Enriched with a mass of closely observed 

 phenomena and strictly deduced generalisations, this branch of 

 chemistry has been treated separately for the reason that in it the 

 hydrocarbon groups .are subjected to transformations which are not 

 met with in such quantity in dealing with any of the other elements 

 or their hydrogen compounds. It was important for us to show that 

 notwithstanding the great variety of the hydrocarbons and their 

 products, 45 they are all of them governed by the law of substitution, 

 and referring our readers for detailed information to works on organic 

 chemistry, we will limit ourselves to a short exposition of the properties 

 of the two simplest unsaturated hydrocarbons : ethylene, CH 2 CH 2 > and 

 acetylene, CHCH, and a short acquaintance with petroleum as the natu- 

 ral source of a mass of hydrocarbons, Ethylene t or olefiant gas, 2 H 4 , 



43 The following are the most generally known of the oxygenised but non-nitro- 

 genous hydocarbon derivatives. (1) The alcohols. These are hydrocarbons in which 

 hydrogen is exchanged for hydroxyl (OH). The simplest of these is methyl alcohol, 

 CH^OH), or wood spirit obtained by the dry distillation of wood. The common spirits 

 of wine or ethyl alcohol, C 2 H 5 (OH), and glycol, 2 H 4 (OH) 2 , correspond with ethane. 

 Normal propyl alcohol, CH 3 CH 2 CH 2 (OH), and isopropyl alcohol, CH 5 CH(OH)CH 3 , pro- 

 pylene-glycol, C 3 H 6 (OH) 2 , and glycerol, C 3 H 5 (OH) 5 (which, with stearic and other acids, 

 forms fatty substances), correspond with propane, C 3 H S . All alcohols are capable of form- 

 ing water and ethereal salts with acids, just as alkalis form ordinary salts. (2) Aldehydes 

 are alcohols minus hydrogen ; for instance, acetaldehyde, C 2 H 4 O, corresponds with ethyl 

 alcohol. (3) It is simplest to regard organic acids as hydrocarbons in which hydrogen 

 has been exchanged for carboxyl (CO 2 H), as will be explained in the following chapter. 

 There are a number of intermediate compounds ; for example, the aldehyde-alcohols, 

 alcohol-acids (or hydroxy-acids), &c. Thus the hydroxy-acids are hydrocarbons in which 

 some of the hydrogen has been replaced by hydroxyl, and some by carboxyl ; for 

 instance, lactic acid corresponds with C 2 Ho, and has the constitution C 2 H 4 (OH) (C0 2 H). 

 If to these products we add the haloid salts (where H is replaced by Cl, Br, I), the nitro- 

 compounds containing NO 2 in place of H, the amides, cyanides, ketones, and other com- 

 pounds, it will be readily seen what an immense number of organic compounds there are 

 and what a variety of properties these substances have ; this we see also from the com- 

 position of plants and animals. 



