CONSERVATION OF ENERGY 235 



tions urged at the commencement of the chapter 

 as to the real nature and significance of the 

 term " Law of Nature," and to remember that 

 this really means a generalisation from the facts 

 before us up to date. " The term 'energy ' Energy 

 itself," says Sir Oliver Lodge,* " as used in 

 a definite sense by the physicist, rather involves 

 a modern idea, and is itself a generalisation. 

 Things as distinct from each other as light, heat, 

 sound, rotation, vibration, elastic strain, gravi- 

 tative separation, electric currents, and chemi- 

 cal affinity, have all to be generalised under 

 the same heading (of the Conservation of 

 Energy) in order to make the law true. Until 

 6 heat ' was included in the list of energies, the 

 statement could not be made ; and a short time 

 ago it was sometimes' discussed whether 6 life ' 

 should or should not be included in the category 

 of energy. I should give the answer decidedly 

 No, but some might be inclined to say Yes ; and 

 this is sufficient as an example to show that the 

 categories of energy are not necessarily ex- 

 hausted; that new forms may be discovered; 

 and that if new forms exist, until they 

 are discovered, the Law of Conservation 

 of Energy, as now stated, may in some 

 cases be strictly untrue, though partially and 

 isefully true; just as it would be untrue, 



* Life and Matter, p. 21. 



