THE ENERGY EQUATION. 393 



more satisfactorily defined as the one hundredth part of the 

 quantity of heat required to raise the temperature of a gram 

 of water from o to 100 C. This gives the ordinary or small 

 calorie. In dealing with large heat transfers a larger unit is 

 preferable and one just 1000 times as large is frequently used. 

 In this the kilogram in place of the gram of water is warmed, 

 and the unit is called the large calorie. The first may be ab- 

 breviated cal. and the second Cal. 



Unit of Work and Unit of Force. The unit of force is 

 called the dyne and may be defined as the force which, acting 

 for i second on a mass of i gram, gives to it an acceleration 

 of i centimeter per second. The force of gravity at the sea 

 level is about 981 dynes, since this adds to a falling body an 

 acceleration of 981 cm. per second. 



The unit of icork is the erg, and it may be defined as the 

 work done in overcoming unit force through unit distance. 

 One dyne acting through one centimeter gives us one erg of 

 work. To lift i gram through i centimeter requires 981 ergs 

 of work. 



Mechanical Equivalent of Heat. Work may be done by 

 the proper utilization of heat, and in turn work may be wholly 

 converted into heat. It is possible therefore to express one 

 in terms of the other. The mechanical or work equivalent 

 of a unit of heat has been determined many times by very 

 elaborate experiments. If a given quantity of heat could be 

 applied wholly to the lifting of a weight it would be found, in 

 accordance with the mean results of these experiments, that i 

 calorie would be able to lift 423.5 gm. through i meter, or i 

 gm. of substance through 423.5 meters. Conversely, if a 

 gram of water be dropped from a height of 423.5 meters, and 

 its energy of motion wholly converted into heat, its tempera- 

 ture will be found to be increased i C. We have then these 

 relations : 



i calorie = 42,350 gm. cm. 

 = 41,500,000 ergs. 



Heats of Combustion. By means of calorimeter experi- 



