CHAPTEE XIX. 



EXPERIMENT. 



WE now come to consider the great facilities which we 

 enjoy for examining the possible combinations of proper- 

 ties and phenomena when objects are within our reach 

 and capable of manipulation. We are said to experiment, 

 when we bring substances together under various con- 

 ditions of temperature, pressure, electric disturbance, 

 molecular attraction, &c., and then record the changes 

 observed. 



If we denote by A a certain group of antecedent con- 

 ditions, and by X a certain series of subsequent phe- 

 nomena, our object will usually be to ascertain a law of 

 the form A = AX, the meaning of which is that where A is 

 X will happen, and we may sometimes rise to the still 

 simpler and higher law A = X, meaning that where A is, 

 and only where A is, X will happen (see vol. i. pp. 146, 

 149.) 



The great object of the art of experiment is to ascertain 

 exactly those circumstances or conditions which are re- 

 quisite for the happening of any event X. Now the cir- 

 cumstances which might be enumerated as present in the 

 very simplest experiment are very numerous, in fact 

 almost infinite. Bub two sticks together and consider 

 what would be an exhaustive statement of the conditions. 

 There are the form, hardness, organic structure, and all 

 the chemical qualities of the wood ; the pressure and velo- 

 city of the rubbing ; the temperature, pressure, and all the 

 chemical qualities of the surrounding air ; the proximity 



