PHYSICAL SCIENCE 39 



all possible conditions in all possible depths of the 

 universe. 



Mechanical and electrical work may be transformed 

 completely into heat by means of friction or resist- 

 ance ; but the reverse operation can only be com- 

 pleted in very special circumstances. The conditions 

 of the limited change which is usually alone possible 

 are of great importance in the theory of heat engines, 

 and are the subject of that branch of science known 

 as thermodynamics. All heat engines must possess 

 a hot body or source of heat and a cold body or 

 condenser, and the transformation of heat into work 

 will be more complete, and the engine therefore 

 more efficient, the greater the difference of tempe- 

 rature between the source and the condenser. The 

 change can only be complete if the temperature 

 ratio be infinity that is, if the temperature of the 

 condenser be zero. Hence is reached the concep- 

 tion of a true absolute zero of temperature, un- 

 connected with the properties of any particular 

 thermometric substance like mercury, a zero which 

 has been approached more and more nearly as one 

 gas after the other has been liquefied. 



Simultaneously with this development in the 

 science of heat, a great advance was made in the 

 kindred subject of light. Newton was the first to 

 give a satisfactory demonstration of the really com- 

 plex nature of white light, and to resolve it completely 



