468 Prof. Challis on the Fundamental Ideas of 



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 understands and acts upon, as to how a body may be 

 moved ; and the rule of philosophy which makes personal sen- 

 sation and experience the basis of scientific knowledge, as they 

 are the basis of the knowledge that regulates the common trans- 

 actions 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 invisible, 

 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 philo- 

 sophy I am advocating, which assume that the information of the 

 senses is adequate, with the aid of mathematical reasoning, to 

 explain phenomena of all kinds. Probably it will be urged, in 

 opposition to these views, that in cases of personal sensation of 

 pressure, as when the hand presses against any substance, no 

 parts of the two substances are really in contact, the molecular 

 forces keeping the atoms asunder. I allow this to be the case, 

 and still maintain that the idea of pressure by contact is di- 

 stinctly conveyed by sensational experience, and that this idea, 

 united with the consciousness of being able to move a body by 

 pressure, is our only means of understanding cause of motion. 

 Consequently pressure by contact, even though it be an abstrac- 

 tion, is the appropriate basis of scientific research respecting the 

 action and the laws of forces. 



(7) From the hypothesis that the atoms of bodies are spheres 

 of constant magnitude, it follows that they have the property of 

 resisting any pressure tending to change their form or mag- 

 nitude. The pressure being supposed to be caused by the con- 

 tact of an elastic fluid medium, the resistance acts as a pressure 

 on the medium just equal and opposite to that of the medium 

 on the atom. This power of resistance is to be regarded as an 

 essential quality of the atom, implied in constancy of form and 

 magnitude, and, like inertia, an ultimate fact, for which no 

 antecedent cause is assignable. It acts, however, in a manner 

 which is perfectly intelligible from common experience; and as 

 its action takes place only when brought into play by the pres- 

 sure of an external substance, I propose to call it atomic re- 

 action. 



(8) All physical force being pressure, there must be a me- 

 dium by which the pressure is exerted. The antecedents of 

 physical science have in various ways led to the hypothesis of 

 an ^ther. The idea of this medium was fully entertained by 



