THE LAW OF SUBSTANCE. 217 



produces all phenomena, is unchangeable. When the 

 locomotive rushes along the line, the potential energy 

 of the steam is transformed into the kinetic or actual 

 energy of the mechanical movement ; when we hear 

 its shrill whistle, as it speeds along, the sound-waves 

 of the vibrating atmosphere are conveyed through the 

 tympanum and the three bones of the ear into the 

 inner labyrinth, and thence transferred by the auditory 

 nerve to the acoustic ganglionic cells which form the 

 centre of hearing in the temporal lobe of the grey 

 bed of the brain. The whole marvellous panorama 

 of life that spreads over the surface of our globe is, 

 in the last analysis, transformed sun-light. It is well 

 known how the remarkable progress of technical 

 science has made it possible for us to convert the 

 different physical forces from one form to another ; 

 heat may be changed into molar movement, or move- 

 ment of mass; this in turn into light or sound, 

 and then into electricity, and so forth. Accurate 

 measurement of the quantity of force which is used 

 in this metamorphosis has shown that it is " constant " 

 or unchanged. No particle of living energy is ever 

 extinguished ; no particle is ever created anew. 

 Friedrich Mohr, of Bonn, was very near to the 

 discovery of this great fact in 1837, but the discovery 

 was actually made by the able Swabian physician, 

 Robert Mayer of Heilbronn, in 1842. Independently 

 of Mayer, however, the principle was reached almost 

 at the same time by the famous physiologist, Hermann 

 Helmholtz ; five years afterwards he pointed out its 

 general application to, and fertility in, every branch 

 of physics. We ought to say to-day that it rules also 

 in the entire province of physiology — that is, of 

 " organic physics "; but on that point we meet a 



