CHAP. XIX.] EXPERIMENT. 417 



unsafe to assume that any one of these circumstances is 

 without effect, and it is only by experience that we can 

 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 pre- 

 sence 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 w^e may say that 

 air is not,- in the presence of the other circumstances, a 

 requisite- condition. The conduction of heat from neigh- 

 bouring bodies may be a condition. Prevent this by mak- 

 ing all the surrounding bodies ice cold, which is 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 expen- 

 diture of energy in the friction of two bodies is the sole 

 condition of the production of heat. 



The great difficulty of experiment arises from the fact 

 that we must not assume the conditions to be independent. 

 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, and in another when air 

 is absent. The inquiry branches out into two lines, and 

 we ought to try in both cases whether cutting off a supply 

 of heat by conduction prevents its evolution in friction. 

 The same branching out of the inquiry occurs with regard 

 to every circumstance whicli enters into the experiment. 



Regarding only four circumstances, say A, B, C, D, we 

 ought to test not only the combinations ABCD, ABC^^, 

 ABcD, AZ/CD, aBCD, but we ought really to go through 

 the whole of the combinations given in the fifth column 

 of the Logical Alphabet. The effect of the absence of 

 each condition should be tried both in the presence and 

 absence of every other condition, and every selection of 

 those conditions. Perfect and exliaustive experimentation 

 would, in short, con.sist in examining natural phenomena 

 in all their possible combinations and registering all 



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