220 TRANSFORMATION AND EQUIVALENCE OF FORCES. 



that the forces which we distinguish as mental, come within 

 the same generalization. Yet there is no alternative but to 

 make this assertion: the facts which justify, or rather which 

 necessitate it, being abundant and conspicuous. They fall 

 into the following groups. 



All impressions from moment to moment made on our 

 organs of sense, stand in direct correlation with physical 

 forces existing externally. The modes of consciousness 

 called pressure, motion, sound, light, heat, are effects pro- 

 duced in us by agencies which, as otherwise expended, crush 

 or fracture pieces of matter, generate vibrations in surround- 

 ing objects, cause chemical combinations, and reduce sub- 

 stances from a solid to a liquid form. Hence if we regard 

 the changes of relative position, of aggregation, or of chem- 

 ical state, thus arising, as being transformed manifestations 

 of the agencies from which they arise ; so must we regard the 

 sensations which such agencies produce in us, as new forms 

 of the forces producing them. Any hesitation to admit 



that, between the physical forces and the sensations there 

 exists a correlation like that between the physical forces 

 themselves, must disappear on remembering how the one 

 correlation, like the other, is not qualitative only but quanti- 

 tative. Masses of matter which, by scales or dynamo- 

 meter, are shown to differ greatly in weight, differ as greatly 

 in the feelings of pressure they produce on our bodies. In 

 arresting moving objects, the strains we are conscious of are 

 proportionate to the momenta of such objects as otherwise 

 measured. Under like conditions the impressions of sounds 

 given to us by vibrating strings, bells, or columns of air, are 

 found to vary in strength with the amount of force applied. 

 Fluids or solids proved to be markedly contrasted in tem- 

 perature by the different degrees of expansion they produce 

 in the mercurial column, produce in us correspondingly dif- 

 ferent degrees of the sensation of heat. And similarly un- 

 like intensities in our impressions of light, answer to un- 

 like effects as measured by photometers. 



