SLOW AND BAPID REACTIONS. 39 



to a shock capable of producing an explosion which is then 

 gradually propagated. 



Let us first examine the former case. We shall distinguish 

 the molecular rapidity of the reactions, which is defined by the 

 quantity of matter transformed at a fixed temperature and con- 

 stant pressure, under invariable conditions, and the rapidity of 

 propagation of reactions. 



For greater clearness the phenomena will be treated in a 

 general manner in the first section, then the molecular rapidity 

 of reactions in a homogeneous system, submitted to uniform 

 conditions and contained in an enclosure to which it cannot 

 yield, and from which it cannot borrow heat, will be specially 

 studied (second section). Lastly, a system also homogeneous, 

 but which can lose heat, will be examined (third section). 



2. Suppose, first, a certain body, or a certain mixture, capable 

 of undergoing a chemical transformation. When the whole 

 mass is placed under the same conditions of temperature, 

 pressure, or of vibratory movement, etc., it seems as if the 

 reaction must be instantaneously developed in all the parts at 

 the same time ; the sudden explosions of nitrogen chloride and 

 of nitroglycerin would seem favourable to this idea at first 

 sight. However, a closer observation proves that molecular 

 reactions generally require a certain time for their accomplish- 

 ment, even when they liberate heat. Such is, for instance, the 

 decomposition of formic acid into hydrogen and carbonic acid, 

 which can be easily followed experimentally owing to the 

 slowness with which this decomposition is effected. Carried 

 out in a closed vessel and maintained at the fixed temperature 

 of 260, it requires many hours. Nevertheless this reaction 

 liberates 5800 cal. per equivalent of formic acid, viz. 126 cal. 

 per gramme. 1 * 



3. The following are further examples of reactions liberating 

 a large quantity of heat, without, however, being instantaneous. 



Thus, acetylene, changed into benzene towards a dull red heat 

 by a slow reaction, liberates at the same volume half as much 

 heat again as a detonating mixture formed of oxygen and 

 hydrogen in the proportions of water, viz. 85,500 cal. for 

 33-6 litres of acetylene (reduced to and 0-760), instead of 

 59,000 cal., produced by the formation of gaseous water by 

 means of the same volume of electrolytic gas. 



It is about four times as much as the heat liberated by a 

 chlorate powder, weight for weight, viz. 2192 cal. per 1 grm. of 

 transformed acetylene, instead of 590'6 cal. for 1 grm. of 

 potassium chlorate powder. 



Cyanogen liberates three times as much (1435 cal. per 1 grm.) 

 as the same weight of chlorate powder, and this number is 



1 " Essai de M&anique Chimique," torn. ii. p. 17, and especially p. 58 and 

 following. 



