Dr. J. Groll on the Transformation of Gravity \ 253 



be a force without calling in the aid of corpuscles or a medium 

 filling space ? 



On a former occasion * I endeavoured to show at consider- 

 able length that our inability to conceive how force can exist 

 without a material medium has its foundation in a meta- 

 physical misconception. 



Force cannot be conceived to exist of itself ; for it is not a 

 substance, but must be either the property of a substance or 

 the effect of a cause. But there is no a priori necessity for 

 forming any conception of the nature of the substance of 

 which it is the property, or of the cause of which it is the 

 effect. This is not all. To suppose that the substance of which 

 the force is the property should be some material tangible 

 thing such as a corpuscle, is to suppose that Ave must not only 

 know the nature of the substance, but should also be able to 

 form a sensuous representation of it. Here is a double error ; 

 for it is only the properties that are cognizable through the 

 senses ; or, in other words, we know substance only through 

 its properties, or in the way that it manifests itself. Every 

 thing in the corpuscle, for example, which is manifested to 

 the eye, to the ear, or to the touch, is just as much a property 

 or an effect as is force itself. All that the intellect demands 

 is that the force of gravity be the force of something', but what 

 that something is other than a something manifesting itself as 

 force we have no necessity for knowing. It does not follow 

 that this something should have the additional properties of 

 hardness, length, breadth, and thickness, &c, so that it may 

 be designated a corpuscle, a material particle, or an elastic 

 medium &c. 



Gravity as a Retarding Cause. — I shall now briefly refer to 

 a curious consequence which appears to follow from the im- 

 pact theory, independently of any considerations which have 

 been here advanced regarding the transformation of gravity. 

 If gravity results from corpuscular impact, it follows, although 

 the contrary seems to have been generally assumed, that the 

 force of impact will be greater when a body is moving in 

 opposition to the corpuscles than when moving in the same 

 direction, unless we assume, what would be absurd, that they 

 move with infinite velocity. The collision in the case of two 

 trains meeting each other is more severe than in the case of 

 the one overtaking the other. 



It therefore follows that, even though the atmosphere offered 



no resistance, a body projected vertically upwards would not 



return to the earth with absolutely the same velocity as it left. 



A stone, for example, projected upwards against an excessively 



* riulo?ophy of Theism. Ward and Co. : 1857. 



