250 THE PRINCIPLES OF SCIENCE. 



The very use of science consists in enabling us to act with 

 confidence, because we can foresee the result. Now this 

 foresight must rest upon the knowledge of the powers 

 which will come into play. That knowledge, indeed, can 

 never be certain, because it rests upon imperfect induc- 

 tion, and the most confident beliefs and predictions of the 

 physicist may be falsified. Nevertheless, if we always 

 estimate the probability of each belief according to the 

 due teaching of the data, and bear in mind that probability 

 when forming our anticipations, we shall ensure the mini- 

 mum of disappointment. Even when he cannot exactly 

 apply the theory of probabilities, the physicist may acquire 

 the habit of making judgments in general agreement with 

 its principles and results. 



Such is the constitution of nature, that the physicist 

 soon learns to distinguish those properties which have 

 wide and uniform extension, from those which vary 

 between case and case. Not only are certain laws dis- 

 tinctly laid down, with their extension carefully defined, 

 but a scientific training gives a kind of tact in judging 

 how far other laws are likely to apply under any parti- 

 cular circumstances. We learn by degrees that crystals 

 exhibit phenomena depending upon the directions of the 

 axes of elasticity, which we must not expect in uniform 

 solids. Liquids, compared even with non -crystalline 

 solids, exhibit laws of far less complexity and variety ; 

 and gases assume, in many respects, an aspect of nearly 

 complete uniformity. To trace out the branches of science 

 in which varying degrees of generality prevail, would be 

 found to be an inquiry of great interest and importance ; 

 but want of space, if there were no other reason, would 

 forbid me to attempt it, except in a very slight manner. 



Gases, so far as they are really gaseous, not only have ex- 

 actly the same properties in all directions of space, but one 

 gas exactly resembles other gases in a great many qualities. 



