4 PWNCIPLES OF CHEMISTRY 



the gases and vapours contained in the smoke being the products of 

 combination. 



2. Reactions of decomposition are cases the reverse to those of 

 combination, that is, in which one substance gives two or, in general, a 

 given number of substances a greater number. Thus, by heating wood 

 (and also coal and many animal or vegetable substances) without access 

 to air, a combustible gas, a watery liquid, tar, and carbon are obtained. 

 It is in this way that tar, lighting gas, and charcoal are prepared on a 

 large scale. 8 All limestones, for example, flagstones, chalk, or marble, 

 are decomposed by heating to redness into lime and a peculiar gas 

 called carbonic anhydride. A similar decomposition, taking place, 

 however, at a much lower temperature, proceeds with the green copper 

 carbonate which enters into the composition of malachite. This ex- 

 ample will be studied more in detail presently. Whilst heat is evolved 

 in the ordinary reactions of combination, it is, on the contrary, con- 

 sumed in the reactions of decomposition. 



3. The third class of chemical reactions where the number of acting 

 substances is equal to the number of substances formed consists, as it 

 were, of an association of decomposition and combination. If, for 

 instance, two compounds A and B are taken and they react on each 

 other to form the substances C and D, then supposing that A is de- 

 composed into D and E, and that E combines with B to form C, we 

 have a reaction in which two substances A, or D E, and B were taken 

 and two others C, or E B, and D were produced. Such reactions ought 

 to be placed under the general term of reactions of 'rearrangement,' 

 and the particular case where two substances give two fresh ones, 

 reactions of * substitution.' 9 Thus, if a piece of iron be immersed in a 

 solution of blue vitriol (copper sulphate), copper is formed or, rather, 



8 Decomposition of this kind is termed ' dry distillation ' because, as in distillation, 

 the substance is heated and vapours are given off which, on cooling, condense into 

 liquids. In general, decomposition, in absorbing heat, presents much in common to a 

 physical change of state such as, for example, that of a liquid into a gas. Deville 

 likened complete decomposition to boiling, and compared partial decomposition, when a 

 portion of a substance is not decomposed in the presence of its products of decomposition 

 (or dissociation), to evaporation. 



9 A reaction of rearrangement may in certain cases take place with one substance 

 only ; that is to say, a substance may by itself change into a new isomeric form. Thus, 

 for example, if hard yellow sulphur be heated to a temperature of 250 and then poured 

 into cold water it gives, on cooling, a soft, brown variety. Ordinary phosphorus, which 

 is transparent, poisonous, and phosphorescent in the dark (in air), gives, after being 

 heated at 270 (in an atmosphere incapable of supporting combustion, such as steam), an 

 opaque, red, and non-poisonous isomeric variety, which is not phosphorescent. Cases of 

 isomerism point out the possibility of an internal rearrangement in a substance, and are 

 the result of an alteration in the grouping of the same elements, just as a certain number 

 of balls may be grouped in figures and forms of different shapes and of various properties. 



