ADDRESS. 15 



motion, or can this motion explain any of the known phenomena of our 

 science ? I have already said that Lavoisier left untouched the dynamics 

 of combustion. He could not explain why a fixed and unalterable amount 

 of heat is in most cases emitted but in some cases absorbed when 

 chemical combination takes place. What Lavoisier left unexplained 

 Joule has made clear. On August 25, 1843, Joule read a short communi- 

 cation, I am glad to remember, before the Chemical Section of our 

 Association, meeting that year at Cork, containing an announcement of 

 a discovery which was to revolutionise modern science. This consisted 

 in the determination of the mechanical equivalent of heat, in proving by 

 accurate experiment that the expenditure of energy equal to that developed 

 by the weight of 772 pounds falling through one foot at Manchester, the 

 temperature of one pound of water can be raised 1° Fahrenheit. In 

 other words, every change in the arrangement of the particles is accom- 

 panied by a definite evolution or an absorption of heat. In all such cases 

 the molecular energy leaves the potential to assume the kinetic form, 

 or vice versa. Heat is evolved by the clashing of the atoms, and this 

 amount is fixed and definite. 



Thus it is to Joule we owe the foundation of chemical dynamics and the 

 basis of thermal chemistry. As the conservation of mass or the principle 

 of the indestructibility of matter forms the basis of chemical statics, 

 so the principle of the conservation of energy ' constitutes the foundation 

 of chemical dynamics. Change in the form of matter and change in the 

 form of energy are the universal accompaniments of every chemical 

 operation. Here again it is to Joule we owe the proof of the truth ot 

 this principle in another direction, viz., that when electrical energy is 

 developed by chemical change a corresponding quantity of chemical 

 energy disappears. Energy as defined by Maxwell is the power of doing 

 work, and work is the act of producing a change of configuration in a 

 system in opposition to a force which resists that change. Chemical 

 action produces such a change of configuration in the molecules. Hence, 

 as Maxwell says, 'a complete knowledge of the mode in which the 

 potential energy of a system varies with the configuration would enable 

 us to predict every possible motion of the system under the action ot 

 given external forces, provided we were able to overcome the purely 

 mathematical difficulties of the calculation.' The object of thermal 

 chemistry is to measure these changes of energy by thermal methods, 

 and to connect these with chemical changes, to estimate the attractions 

 of the atoms and molecules to which the name of chemical affinity has 

 been applied, and thus to solve the most fundamental problem of 

 chemical science. How far has modern research approached the solution 

 of this most difficult problem ? How far can we answer the question, 



' ' The total ^^energy of any material system is a quantity which can neither be 

 increased nor^diminished by any action between the parts of the system, though it 

 may be transformed into any of the forms of which energy is susceptible.' — Max- 

 well. 



