THE USE OF HYPOTHESIS. 141 



jo far liken it to matter. And as when applying those 

 laws to the elastic medium air, we are able to infer the 

 Dhenomena of sound, so by arguing in a similar manner 

 concerning ether we are able to infer the existence of 

 ight phenomena corresponding to what do occur. All 

 :hat we do is to take a material elastic substance, increase 

 its elasticity in an almost indefinite degree, and denude it 



gravity and some others of the ordinary properties of 

 matter, but we must retain sufficient likeness to matter to 

 allow of deductive calculations. 



The force of gravity is in some respects an almost in- 

 comprehensible existence, but in other respects entirely 

 conformable to experience. We can distinctly observe 

 that the force is proportional to mass, and that it acts in 

 entire independence of the other matter which may be 

 present or intervening. The law of the decrease of in- 

 tensity as the square of the distance increases, may be 

 observed to hold true of light, sound, and any other 

 influences emanating from a point, and spreading uni- 

 formly through space. The law is doubtless connected 

 at this point with the primary properties of space itself, 

 and is so far conformable to our necessary ideas. 



It may well be said, however, that no hypothesis can 

 be so much as framed in the mind unless it be more or 

 less conformable to experience. As the material of our 

 ideas is undoubtedly derived from sensation, so we cannot 

 figure to ourselves any existence or agent, but as endowed 

 with some of the properties of matter. All that the mind 

 can do in the creation of new existences is to alter com- 

 binations, or by analogy to alter the intensity of sensuous 

 properties. The phenomenon of motion is familiar to 

 sight and touch, and different degrees of rapidity are also 

 familiar : we can pass beyond the limits of sense, and 

 suppose the existence of rapid motion, such as our senses 

 could not measure or observe. We know what is elasticity, 



