EXPERIMENT. 23 



of the earth with its attractive and electric properties ; 

 the temperature and other powers of the persons pro- 

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

 from the sky ; the electric excitement possibly existing 

 in any overhanging cloud ; even the positions of the 

 heavenly bodies must be mentioned. Now on a priori 

 grounds it is unsafe to assume that any one of these 

 circumstances is without effect, and it is only on the 

 results of experience that we can finally single out those 

 precise conditions from which the observed heat of friction 

 proceeds. 



The great method of experiment consists in removing, 

 one at a time, each of those conditions which may be 

 imagined to have an influence on the result. Our object 

 in the experiment of rubbing sticks is to discover the 

 exact circumstances under which heat appears. Now the 

 presence of air may be requisite ; therefore prepare a 

 vacuum, and rub the sticks in every respect as before, 

 except that it is done in vacuo. If heat still appears we 

 may say that air is not, in the presence of the other 

 circumstances, a requisite condition. The conduction of 

 heat from neighbouring bodies may be a condition. 

 Prevent this by making all the surrounding bodies ice 

 cold, which is practically what Davy aimed at in rubbing 

 two pieces of ice together. If heat still appears we have 

 eliminated another condition, and so we may go on until 

 it becomes apparent that the expenditure of energy in the 

 friction of two bodies is the sole condition of the produc- 

 tion of heat. 



The great difficulty of experiment arises from the fact 

 that we must not assume an independence to exist among 

 the conditions. Thus previous to experiment we have no 

 right to say that the rubbing of two sticks will produce 

 heat in the same way when air is absent as before. We 

 may have heat produced in one way when air is present, 



