A HISTORY OF SCIENCE 



or gravity could be converted into heat, the process is 

 not fully reversible. Heat can, indeed, be converted 

 into molar motion or work, but in the process a certain 

 amount of the heat is radiated into space and lost. The 

 same thing happens whenever any other form of energy 

 is converted into molar motion. Indeed, every trans- 

 mutation of energy, of whatever character, seems com- 

 plicated by a tendency to develop heat, part of which 

 is lost. This observation led Professor Thomson to his 

 doctrine of the dissipation of energy, which he formu- 

 lated before the Royal Society of Edinburgh in 1852, 

 and published also in the Philosophical Magazine the 

 same year, the title borne being, "On a Universal 

 Tendency in Nature to the Dissipation of Mechanical 

 Energy." 



From the principle here expressed Professor Thom- 

 son drew the startling conclusion that, ' ' since any res- 

 toration of this mechanical energy without more than 

 an equivalent dissipation is impossible," the universe, 

 as known to us, must be in the condition of a machine 

 gradually running down; and in particular that the 

 world we live on has been within a finite time unfit for 

 human habitation, and must again become so within a 

 finite future. This thought seems such a common- 

 place to-day that it is difficult to realize how startling 

 it appeared half a century ago. A generation trained, as 

 ours has been, in the doctrines of the conservation and 

 dissipation of energy as the very alphabet of physical 

 science can but ill appreciate the mental attitude of a 

 generation which for the most part had not even 

 thought it problematical whether the sun could con- 

 tinue to give out heat and light forever. But those 



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