" Regula Tertia PhilosopliandV 25 



sical philosophy, I have further supposed, for the purpose of 

 applying mathematical reasoning, that the atoms are impene- 

 trable spheres, that the aether is a perfect fluid of constant in- 

 trinsic elasticity, and that it varies in pressure in exact pro- 

 portion to variations of its density. The aether is thus defined 1 

 by the equation p = kp,p being the pressure, p the density, 

 and k&n absolute constant. These three hypotheses are of the 

 kind which have to be verified by comparisons of deductions 

 from them with experimental facts. 



Bearing in mind Newton's monition not to make hypotheses 

 contrary to "the tenor of experience," or to deviate from 

 "the analogy of nature," I have added the above-stated hy- 

 potheses on the following grounds. First, since experience 

 has brought to our knowledge no other kind of masses of 

 matter than such as are atomically constituted, and since a 

 universal plenum under any natural conditions is inconceivable, 

 I suppose the aether to have a uniform atomic constitution — 1 

 that is, to consist of minute atoms all of the same size and I 

 uniformly distributed. Under this constitution it may be I 

 conceived to be susceptible of variations of density from point 

 to point, and to be capable of pressing in proportion to the 

 density. Again, in the formula p = kp as applied to the aether, 

 there can be no variation of the factor k, because, as the exist- 

 ence of the aether is supposed to be the primary condition of 

 physical force, there are no ulterior forces to which a variation 

 of its elasticity can be referred. This view makes it neces- 

 sary to demonstrate that all the forms of physical force are 

 modes of pressure of the aetherial medium. To show this has 

 been the express object of physical researches (subsequently 

 referred to in this communication) on which I have been en- 

 gaged during many years. Further, it is to be said that a 

 system of philosophy which rests wholly on the indications of 

 sensation and experience can admit of no other kind of force 

 than pressure, because of this force we have immediate sensible 

 cognition (as when we press with the hand against any sub- , 

 stance) ; and the same assertion cannot be made respecting 

 any other kind of force. In support of the hypothesis of the 

 spherical form of the atom I might cite another of Newton's 

 sayings, " natura solet esse simplex" the form of the sphere 

 being defined by a single constant. Also the fact that a mass 

 of water remains the same in magnitude and quality after any 

 amount of change of the relative positions of its parts, implies 

 that each ultimate part has the same relation to surrounding 

 parts after as before the disturbance, and that the relation is 

 the same in all directions from the ultimate part. These views 

 may be regarded as justifying the hypothesis of the spherical 



