96 THE POPULAR SCIENCE MONTHLY. 



says Prof. Challis, " than pressure by contact of one body with an- 

 other. This hypothesis is made on the principle of admitting no fun- 

 damental ideas that are not referable to sensation and experience. It 

 is true that we see bodies obeying the influence of an external force, 

 as when a body descends toward the earth by the action of gravity ; 

 and, so far as the sense of sight informs us, we do not in such cases 

 perceive either the contact or the pressure of another body. But we 

 have also the sense of touch or of pressure by contact — for instance, 

 of the hand with another body — and we feel in ourselves the power 

 of causing motion by such pressure. The consciousness of this power 

 and the sense of touch give a distinct idea, such as all the world un- 

 derstands and acts upon, as to how a body may be moved ; and the 

 rule of philosophy which makes personal sensation and experience the 

 basis of scientific knowledge, as they are the basis of the knowledge 

 that regulates the common transactions of life, forbids recognizing any 

 other mode of moving a body than this. When, therefore, a body is 

 caused to move without apparent contact and pressure of another 

 body, it must still be concluded that the pressing body, although in- 

 visible, exists, unless we are prepared to admit that there are physical 

 operations which are and ever will be incomprehensible by us. This 

 admission is incompatible with the principles of the philosophy I am 

 advocating, which assume that the information of the senses is ade- 

 quate, with the aid of mathematical reasoning, to explain phenomena 

 of all kinds. . . . All physical force being pressure, there must be a 

 medium by which the pressure is exerted." 



It is not my purpose, of course, to question the Newtonian doc- 

 trine of gravitation, or to urge the adoption of the views of Prof. 

 Challis and others, who seek to show that what seems to be attraction 

 is in reality a propulsion of solid bodies in immediate contact. That 

 the transfer of motion from one body to another by impact is no less 

 incomprehensible than actio in distans becomes apparent on a mo- 

 ment's reflection ; and that the hypothesis of an intervening " ether " 

 — itself composed of atoms, the interspaces between which are larger 

 in proportion to these atoms than the interstellar spaces — is simply a 

 new presentation of the old perplexity in a worse form, and in no wise 

 helps to remove the difficulties involved in the phenomenon of the cor- 

 respondence between the movements of two bodies without contact, is 

 equally clear, and has been sufficiently pointed out by Herbert Spencer 

 (" First Principles," p. 59). My object is merely to show that, if the 

 validity of every theory of the constitution of matter is to be tested 

 by our ability to realize it in thought — to bring it clearly before the 

 scientific imagination, to represent it mentally as a distinct image, or 

 whatever may be the form of words in which this requirement is ex- 

 pressed — the atomic theory fails as completely as any other theory of 

 the nature of matter which has ever been propounded. 



But what ground is there for the assumption that conceivability is 



