ENERGY AND CHEMICAL CHANGE 161 



basing itself on these facts, affords a means of classifying them 

 and supplies us with a new and useful hypothesis regarding mat- 

 ter in its various forms (iron, sulphur, etc.) and the energy con- 

 tained therein. A brief discussion of such a hypothesis will be 

 found in the concluding chapter (pp. 552-5). 



The Speed of Chemical Actions: a Means of Measuring 

 Activity. One means of measuring the relative chemical activi- 

 ties of several substances is to observe the speed with which they 

 undergo the same chemical change. Thus we may compare the 

 activities of the various metals by allowing them separately to 

 interact with hydrochloric acid and collecting and measuring the 

 hydrogen liberated per minute by each. It will be seen, even in 

 the roughest experiment, that magnesium is thus much more 

 active than zinc. The comparison must be made with such pre- 

 cautions, however, as will make it certain that the conditions 

 under which the several metals act are all alike. Thus, in spite 

 of the heat evolved by the action, means must be used, by suitable 

 cooling, to keep the temperature at some fixed point during the 

 experiment, for all actions become more rapid when the tempera- 

 ture rises (p. 27). Again, the pieces of the various metals must 

 be arranged so that equal surfaces are exposed to the acid in each 

 case. It is found that the order in which this comparison places 

 the metals is much the same as that in which they are placed by 

 a study of other similar actions. A single table, showing the 

 order of activity (p. 54), suffices, therefore, for all purposes. 



Thermochemistry. Chemical changes in which heat, is 

 liberated are called exothermal. Those in which Jheat is continu- 

 ously absorbed (pp. 15, 30) are called endothermal changes. Since 

 the activities, or affinities of two substances (say, two metals) 

 may often be compared by observing the amounts of heat liber- 

 ated when each combines with a third substance (say, oxygen), 

 it will be instructive now to consider some of the elementary facts 

 of thermochemistry. 



