36 PRINCIPLES OF CHEMISTRY 



into mercury and oxygen, and the decomposition of wood under the 

 influence of heat. Many substances are decomposed at a very mode- 

 rate temperature ; for instance, the fulminating salt which is employed 

 in cartridges is decomposed at a little above 120. The majority of 

 those compounds which make up the mass of animal and vegetable 

 matters are decomposed at 250. On the other hand, there is reason 

 to think that at a very low temperature no reaction whatever can 

 take place. Thus plants cease to carry on their chemical processes 

 during the winter. Every chemical reaction requires certain limits 

 of temperature for its accomplishment, and, doubtless, many of the 

 chemical changes observed by us cannot take place in the sun, where 

 the temperature is very high, or on the moon, where it is very low. 



The influence of heat on reversible reactions is particularly instruc- 

 tive. If, for instance, a compound which is capable of being reproduced 

 from its products of decomposition be heated up to the temperature at 

 which decomposition begins, the decomposition of a mass of the sub- 

 stance contained in a definite volume is not immediately completed. 

 Only a certain fraction of the substance is decomposed, the other por- 

 tion remaining unchanged, and if the temperature be raised, the quan- 

 tity of the substance decomposed increases ; furthermore, for a given 

 volume the ratio between the part decomposed and the part unaltered 

 corresponds with each definite rise in temperature until it reaches that 

 at which the compound is entirely decomposed. This partial decom- 

 position under the influence of heat is called dissociation. It is pos- 

 sible to distinguish between the temperatures at which dissociation 

 begins and ends. Should dissociation proceed at a certain temperature, 

 yet should the product or products of decomposition not remain in 

 contact with the still undecomposed portion of the compound, then 

 decomposition will go on to the end. Thus limestone is decomposed 

 in a limekiln into lime and carbonic anhydride, because the latter is 

 carried off by the draught of the furnace. But if a certain mass of 

 limestone be enclosed in a definite volume for instance, in a gun 

 barrel which is then sealed up, and heated to redness, then, as the 

 carbonic anhydride cannot escape, a certain proportion only of the 

 limestone will be decomposed for every increment of heat (rise in tem- 

 perature) higher than that at which dissociation begins. Decomposition 

 will cease when the carbonic anhydride evolved presents a maximum 

 dissociation pressure corresponding with each rise in temperature. If 

 the pressure be increased by increasing the quantity of gas, then com-" 

 bination begins afresh ; if the pressure be diminished decomposition 

 will recommence. Decomposition in this case is exactly similar to 

 evaporation ; if the steam given off by evaporation cannot escape, its 



