Mr. S. T. Preston on the Kinetic Theory of Gravitation. 299 



force, thus explaining " cohesion "*. But then a difficulty at 

 once presents itself here. When two masses (or molecules) 

 are gradually approached towards each other, instead of the 

 tendency to approach gradually increasing up to a maximum 

 (as we should expect from the theory), they begin to repel at 

 a certain distance, and very considerable force is in general 

 required to overcome this first repulsion, when the masses 

 then unite into one. Thus two freshly cut pieces of lead may 

 be made, to unite with some pressure, also glass, or various 

 metals, with more or less pressure. There is therefore a neutral 

 point which has to be passed, when the tendency to recede 

 changes into a tendency to approach. The same thing is ex- 

 hibited (conversely) when a substance is broken into two 

 parts by tension. If pulled (nearly) up to the neutral point, 

 the two parts recoil or return into their old positions. If pulled 

 beyond the neutral point, the parts repel and will not return 

 into their old positions, i. e. they separate permanently. The 

 thing, therefore, to be explained is the existence of this neutral 

 point, or, in other words, the repulsion that exists at a certain 

 distance from the surfaces. 



3. The explanation we have to offer here depends upon quite 

 recent investigations. It must be observed first that facts 

 prove the existence of a second medium in space besides the 

 gravific medium, viz. the heat- or light-conveying medium (the 

 aether). If we admit the existence of one medium in space 

 constituted according to the kinetic theory (the gravific 

 medium), it would be natural to conclude that the second 

 medium (or aether) was constituted in an analogous manner. 

 We shall give independent reasons afterwards that lead to 

 infer this constitution, and endeavour to answer possible objec- 

 tions ; but in the mean time it is only necessary to suppose it 

 to be so constituted (in the absence of proof to the contrary) ; 

 and if this supposition serves to explain in general principle a 

 number of facts, this will be one argument for its truth. On 

 account of the extreme shortness of the waves of light and 

 heat, it would be reasonable to suppose that the length of free 



* The spectroscope proves molecules to "be complex foodies, on account 

 of the number of different periods of vibration they can take up ; and it 

 was pointed out in the last paper that there are grounds for inferring them 

 to possess interstices, or a more or less open structure. It is evident, 

 therefore, that the shapes of molecules, as to whether their parts fitted 

 over each other or not (and thus afforded more or less shelter from the 

 impinging particles of the gravific medium), would have some influence 

 on the behaviour of molecules as to the energy of their approach (reac- 

 tions). This might account in some degree for the varied behaviour 

 termed " chemical affinity," though possibly there are, besides this, other 

 modifyiug physical conditions. 



