CHAPTEE XIX. 



EXPERIMEiSIT. 



We may now consider tlie great advantages which we 

 enjoy in examining the combinations of phenomena when 

 things are within our reach and capable of being experi- 

 mented on. We are said to experiment when we bring sub- 

 stances together under various conditions of temperature, 

 pressure, electric disturbance, chemical action, &c., and 

 then record the chauges observed. Our object in induc- 

 tive investigation is to ascertain exactly the group of cir- 

 cumstances or conditions which being present, a certain 

 other group of phenomena will follow. If we denote by 

 A the autecedeut group, and by X subsequent pheno- 

 mena, our object will usually be to discover a law of the 

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

 will happen. 



The circumstances which might be enumerated as present 

 in the simplest experiment are very numerous, in fact al- 

 most infinite. Paib two sticks touether 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 aud 

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

 tlie chemical qualities of the surrounding air ; tlie proxi- 

 mity of the earth with its attractive and electric powers; 

 the temperature and other properties of the persons pro- 

 ducing motion ; tlie radiation from the sun, and to and 

 from the sky ; the electric excitement possibly existing in 

 any overhanging cloud ; even the positions of the lieavenly 

 bodies must be mentioned. On a priori, grounds it is 



