.... 



Of the Kinetic Theory of Gas. 363 



will do well* to eschew this somewhat convenient, but by no 

 means necessary, encumbrance upon the unadulterated pro- 

 cess of physical investigation which treats the sensible objects 

 themselves, the bare syntheta of perceptions and ultra-per- 

 ceptions, as though they were what brought about the changes 

 that occur in nature ; and which exclusively occupies itself 

 in tracing out the laws that must, under this hypothesis, be 

 in operation in order that the effects may be what they are. 



Another and a very useful scaffolding which helps us in 

 building up our investigation, is the introduction of forces 

 between the physical cause (which is always the vicinity of 

 some natural object) and the effect to be attributed to it 

 under the physical hypothesis. We are thus enabled to speak 

 of the acceleration of a stone in its fall towards the earth 

 either as being due to the neigbourhood of the earth, or as 

 being caused by a force of gravitation which acts on it, which 

 force is, in its turn, regarded as brought into existence by 

 the proximity of the earth to the stone. The introduction of 

 this piece of intermediate scaffolding is of great service — 



1. Because the force can be represented by a line whose 

 length accurately represents the intensity, and whose direction 

 accurately represents the direction of the effect upon the stone 

 of the vicinity of the earth ; 



2. Because the same effect upon the stone might have been 

 due to other physical causes, as, for example, to a spring 

 urging it forward, in which case the same piece of scaffolding, 

 a force represented by the same line in the same position, 

 would occupy its place between the cause and the effect ; 

 and 



3. Because the effect might have been different, while the 

 physical cause remained the same — thus, if the stone lay on 

 the ground, what the vicinity of the earth would have occa- 

 sioned is stress between the stone and the ground. 



Accordingly by referring effects in nature to the operation 

 of forces, we are enabled in each case to indicate with accuracy 

 the intensity and direction of the effect, without having to 

 specify (a) which of several possible physical causes is the 

 one in operation, or (b) which of the possible kinds of effect 

 is that which is being produced : and this in practice is found 

 to be an immense convenience. 



_ * This course is much to be preferred, because it effectually avoids the 

 risk of throwing dust in our own eyes. The justification of the Physical 

 Hypothesis is its utility, not its truth— its incomparable efficiency as a 

 means of investigating nature; and it is better, though not essential, 

 that students of Physics should make no mistake about a matter of this 

 kind. 



