ENERGETICS 99 



quantity of mechanical work represented by different modes of 

 motion remains invariable. 11 



The energy of a given body is the amount of transferable 

 motion stored up in that body, and is measured by its capacity 

 of producing mechanical work. 



Ostwald thus defines energy: '•Energy is work, all that 

 can be obtained from work, and all that can be changed into 

 work. 11 Different forms of energy may be measured in different 

 ways, but all forms of energy can be measured either in 

 units of mechanical work or in units of heat, in kilogramme- 

 metres or foot-pounds or in calories, according as the energy 

 in question is transformed into mechanical work or into heat. 

 The first principle of energetics, the conservation of energy, 

 may be thus expressed : " Energy is eternal ; none is ever 

 created, and none is ever lost. The quantity of energy 

 in the universe is invariable, and is conserved for ever in its 

 integrity. 11 



The unit by which we measure quantities of heat is the 

 calory, the amount of heat required to raise the temperature 

 of one kilogramme of water one degree Centigrade. 



The practical unit of mechanical work is the kilogramme- 

 metre, the work required to raise the weight of one kilogramme 

 to the height of one metre. The theoretical unit of work is 

 one erg, the work required to move a mass of one gramme 

 through one centimetre against a force of one dyne. 



Joule of Manchester was the first to verify Mayer's law 

 Quantitatively. By an experiment analogous to that of 

 Rumford, he transformed work into heat, arranging his 

 apparatus so that he might measure the amount of heat 

 produced and the work expended. On dividing the quantity 

 of work that had disappeared by the quantity of heat which 

 had been disengaged, he found that 424 kilogramme- 

 metres of work had been expended for each calory of heat 

 produced. 



Him of Colmar measured the ratio of work to heat in the 

 steam engine. He found that for each calory of heat which 

 had disappeared there were produced 425 kilogramme-metres 

 of work. 



