SECOND LAW OF THERMODYNAMICS 231 



Simple Knowledge Assumed 



A great deal of simple every-day knowledge is always taken for 



granted in a treatise on thermodynamics. It is stated above that the 



important things in connection with the generation of steam in a 



boiler by the burning of coal are (a) the temperature of the feed water, 



(b) the temperature and pressure of the steam which is produced, (c) 



the character of the coal, (d) the temperature and composition of the 



air, and (e) the temperature and composition of the flue gases. In 



a certain sense this is true, but of course the fundamentally important 



thing is the knowledge that coal will burn and convert water into 



steam. Such fundamental knowledge is always taken for granted in 



the study of thermodynamics. The nature of fire is not an object of 



study in thermodynamics, but every one knows what fire is in a simple 



practical way; every one knows that an object becomes hot when it 



is placed upon a hot stove; and every one knows that steam will squirt 



out of a hole in a steam boiler under pressure. In the experience of 



the writer only one case has ever come to notice in which this kind 



of fundamental knowledge seemed to be lacking. A student was 



asked to define what is meant by the heat of combustion of coal, and 



he gave it correctly up to a certain point by saying that it was the 



number of thermal units generated by one pound of coal; it was, 



however, impossible to lead the young man by indirect suggestion to 



add the important qualifying phrase "when the coal is burned," and 



upon being asked explicitly how one gets heat out of coal, the young 



man actually replied, with some embarrassment, " Why, Professor, I 



don't know." Of course, he did know, but apparently he could only 



think that the study of thermodynamics must refer to unfamiliar and 



elegant things. No, thermodynamics refers to the things of the 



kitchen and to the things of the furnace, although the science of 



thermodynamics is so organized that it talks only of the things that 



go into and of the things which come out of those mysterious places 



where maids and furnace-men rule. 



Limitations of Mechanics 



The science of mechanics applies to the more or less ideal phe- 

 nomena which are associated with the motion of rigid bodies either 

 singly or in connected machines; with the regular motion of distortion 

 of elastic bodies like the bending of a bow or the oscillation of a string ; 

 and with ideally simple motion of flow of liquids and gases like the 

 smooth flow of water from an orifice in a tank. In every actual case 

 of motion, however, we always encounter turbulence more or less 

 marked, and the science of mechanics, which is the science of describing 

 the phenomena of motion, fails utterly if we attempt to consider the 



