TRANSACTIONS OF THE SECTIONS. 57 



we may speak of a moving body as a mechanical force because it cannot come into 

 contact, be. brought into relation with another body without there being thus a dif- 

 ference of the previously existing polar pressures on that body, and hence a motion 

 or change of motion ; so we may speak of heat, electricity, &c. as forces, because 

 bodies in such states of molecular motion or tension cause a motion of other bodies, 

 or of their molecules. And these forces are thus conceived, not as absolutely exist- 

 ing agents acting on matter, but as conditions of matter. 



It is evident that this idea of force, by which all particular forces become one (by 

 being referred to the same general conception of a difference of pressure), postulates 

 a plenum. But this will probably be now generally granted. If, then, there is a 

 plenum, we may conceive the influence which every part of matter exerts on every 

 other, as acting not " at a distance," but through other intermediate matter, form- 

 ing lines of pressure. Hence Ave may conceive a body or molecule as a centre of 

 pressure, and see whether, retaining the usual mechanical conception of" pressure" 

 as " a balanced force," or " virtual momentum," such influence, or, more definitely, 

 physical and chemical phenomena, become mechanically explicable. 



A molecule, therefore, or body, an aggregate of molecules, is conceived as a centre 

 of lines of pressure ; the lengths and curves of these lines are determined by the rela- 

 tive pressure of the lines they meet ; and lines from greater, are made up of similar 

 lines from lesser molecules, and so on ad infinitum. In speaking of a molecule or 

 body as such a centre of pressure, we may, for convenience sake, call it an atom. 

 In chemistry, the term equivalent will be used exclusively, and not as more or less 

 synonymous with atom, which I have thus ventured to appropriate for a new con- 

 ception. 



Atoms, or mutually determining centres of lines of pressure, may also be defined 

 and mathematically considered as mutually determining elastic systems with centres 

 of resistance. 



But these fundamental conceptions of centres of lines of pressure, or centrally 

 resisting elastic systems, are not hypotheses, but convenient forms of the general 

 conception of the parts of matter as mutually repelling. 



If, in a system of such atoms, the centres, whether molecules or suns, are all of 

 equal mass, and at equal distances, the mutual repulsions of thsir lines will be equal 

 in all directions ; there will be no difference of pressure, no moving force will be 

 developed, and the conditions of equilibrium are satisfied. But it was shown that 

 if there are in such a system differences in the masses of the resisting centres, or 

 in their relative positions, the law of universal attraction, or approach of these 

 centres, whether, in any particular case, equal or unequal, would follow as a me- 

 chanical consequence of the deflection of the mutually opposing lines. 



But such centres may differ not only in mass, but in tension. Tension is conceived 

 as a state of unstable equilibrium, in which a (molecular) atomic centre, having been 

 moved towards the next atom in any plane, rests in a position in which the pressure 

 of its lines is increased in that direction, and correspondingly, of course, diminished 

 in the opposite. Such is the general mechanical conception of polarity proposed in 

 this theory. 



The phenomena caused by bodies in a state of static electricity are deduced from 

 the conception of outward or inward tension iti a closed curve. 



In dynamically electrified bodies the tension is conceived as longitudinal, and the 

 poles as the ends towards and from which the molecules have been moved. A mag- 

 net is conceived as a body in which the molecules are in a permanent state of trans- 

 verse tension ; and hence, evidently, as in Ampere's theory, the analogy (except as 

 to power on iron core) between a helix and a magnet. 



Induction is the necessary mechanical effect on adjacent bodies of electricity or 

 magnetism as above conceived. The character of that effect depends on the rela- 

 tive conditions of the tension of the acting body, and of the mechanical resistance 

 due to the molecular motion or aggregation of the bodies acted on. 



The various motions in the presence of electric or magnetic bodies are explicable 

 as differential effects of (electric or magnetic) conduction. 



It is proposed to apply this general theory to the undulatory theory of light and 

 heat, with the hope that the difficulties therein at present encountered may be hereby 

 overcome. 



