142 Introduction to the Study of Science 



of heat. Water, which has the highest heat capacity of all 

 ordinary substances and is also the most dependable in showing 

 the effects of applied heat, is chosen as the standard substance 

 in the measurement of heat. 



The British Thermal Unit. Those making use, according 

 to the English system of measures, of the pound for a unit 

 quantity of matter, and the degree Fahrenheit for a unit change 

 in temperature, define their heat unit in the same terms. In 

 this system, one pound of water is taken as the unit of weight, 

 and the unit measure of heat is the quantity of heat required to raise 

 one pound of water at 32 F. through one degree. This is known 

 as the British Thermal Unit, commonly abbreviated as B. T. U. 



The Calorie. Those making use of the metric units of 

 measure and the Centigrade scale define their heat unit in a 

 similar manner. One kilogram of water at C. is raised through 

 one degree of temperature by a unit measure of heat called the 

 Calorie. It is practically equivalent to 4 B. T. U. 



The gram calorie. In scientific laboratories and in many 

 textbooks of chemistry and physics another metric unit 

 quantity is used. The amount of heat required to raise one 

 gram of water at zero Centigrade through one degree of tem- 

 perature is called the calorie or often the gram calorie. This 

 is one one-thousandth of the Calorie. 



54. Heat values of fuels. In America fuels have heat values 

 assigned to them in terms of the English system, as so many 

 B. T. U. per pound. A pound of the best grade of anthracite, 

 for example, is said to produce about 14,000 B. T. U. Practi- 

 cally, however, coal is seldom perfectly burned, and some of 

 its heat is lost. But if all the heat from one pound of coal of 

 this grade were applied to 14,000 pounds of pure water, it would 

 effect a rise of one Fahrenheit degree in temperature. 



The practical point here is that in purchasing coal or wood 

 one should consider the heat value together with the cost per 

 ton or cord. For example, there are bituminous coals with a 

 heat value as low as 10,000 B. T. U. per pound, and others that 



