38 PRINCIPLES OF CHEMISTRY 



tions which determine the phenomena' of light is very small, and there- 

 fore only certain, and these generally unstable, compounds can be decom- 

 posed by light at least under ordinary circumstances. But there is 

 one class of chemical phenomena dependent on the action of light 

 which forms as yet an unsolved problem in chemistry these are the 

 processes accomplished in plants under the influence of light. Here 

 there take place most unexpected decompositions and combinations, 

 which are often unattainable by artificial means. For instance, carbonic 

 anhydride, which is so stable under the influence of heat and electricity, 

 is decomposed, and evolves oxygen in plants under the influence of 

 light. In other cases, light decomposes unstable compounds, such as 

 are usually easily decomposed by heat and other agents. Chlorine 

 combines with hydrogen under the influence of light, which shows that 

 combination, as well as decomposition, can be determined by its action, 

 as was likewise the case with heat and electricity. 



(d) Mechanical effects exert, like the foregoing agents, an action 

 both on the process of chemical combination and of decomposition. 

 Many substances are decomposed by friction or by a blow as, for 

 example, the compound called iodide of nitrogen (winch is composed of 

 iodine, nitrogen, and hydrogen), and silver fulminate. Mechanical 

 friction causes sulphur to burn at the expense of the oxygen contained 

 in potassium chlorate. 



(e) Besides the various conditions which have been enumerated 

 above, the progress of chemical reactions is accelerated or retarded by 

 the condition of contact in which the reacting bodies occur. Other 

 conditions remaining constant, the rate of progress of a chemical re- 

 action is accelerated by increasing the number of points of contact. It 

 will be enough to point out the fact that sulphuric acid does not absorb 

 ethylene under ordinary conditions of contact, but only after con- 

 tinued shaking, by which means the number of points of contact is 

 greatly increased. To ensure full action between solids, it is necessary 

 to reduce them to very fine powder and to mix them as thoroughly as 

 possible, as by this means their reaction is greatly accelerated. M. 

 Spring, the Belgian chemist, has shown that finely-powdered solids 

 which do not react on each other at the ordinary temperature may 

 undergo reaction under an increased pressure. Thus, under a pressure 

 of 6,000 atmospheres, sulphur combines with many metals at the ordinary 

 temperature, and the powders of many inetals form alloys. It is evident 

 that an increase in the number of points or surfaces must be regarded 

 as the chief cause producing reaction, which is doubtless accomplished 

 in solids, as in liquids and gases, in virtue of an internal movement or 

 mobility of the particles, which movement, although in different degrees 



