320 PROGRESS IN PHYSICS. 



nor destroyed, although it may appear in many different forms which 

 are in general mutually interconvertible. 



Many men have contributed to the establishment of this great prin- 

 ciple, but it was actually discovered and proved by the labors of three 

 or four. Although it was practically all done before the middle of 

 the nineteenth century, its general popular recognition did not come 

 until a quarter of a century later. The doctrine was first distinctly 

 formulated by Robert Mayer, a German physician, who published in 

 1842 a suggestive paper on "The forces of inorganic nature," which, 

 however, attracted little or no attention. Mayer had not approached 

 the problem from an experimental standpoint. At about the same 

 time it was attacked most successf uU}" from this side by a young Eng- 

 lishman, James Prescott Joule, son of a wealthy brewer of Manchester, 

 England. Joule made the first really accurate determination of the 

 mechanical equivalent of a given quantity of heat, a physical <-onstant 

 which Rumford had tried to measure, reaching only a rough approxi- 

 mation. Substantially Joule's result was that the heat energy ncces- 

 sar}^ to raise the temperature of any given mass of water 1'-' F. is the 

 equivalent of the mechanical energy required to lift that mass through 

 a height of 772 feet against the force of the earth's attraction, and, con- 

 versely, if a mass of water be allowed to fall through a distance of 772 

 feet under the action of gravity and at the end of its motion be instantly 

 arrested, the heat generated will sufiice to raise its temperature 1 F. 

 Of such vast importance is this numerical coeflicient that it has })een 

 called the "golden number" of the nineteenth centmy. Since Joule's 

 time it has been redetermined by several physicists, notabl}' ]>y Pro- 

 fessor Rowland, of Baltimore, the general conclusion being that Joule's 

 number was somewhat, but not greatly, too small. 



The first clear and full exposition of the doctrine of the conserva- 

 tion of energy was given by Joule in a popular lecture in Manchester 

 in 1847, but it attracted little attention until a few months later, when 

 the author presented his theor}- at a meeting of the British Association 

 for the Advancement of Science. Even among scientific men it would 

 have passed without comment or consideration had it not been for the 

 presence of another 3'oung Englishman, then as little known as Joide 

 himself, who began a series of remarks, appreciative and critical, 

 which resulted in making Joule's paper the sensation of the meeting. 

 This was William Thomson, who had been only a year before, at the 

 age of 22 years, appointed professor of natural philosoph}^ at the 

 University of Glasgow, now known as Lord Kelvin, the most versatile, 

 brilliant, and profound student of physical science which the century 

 has produced. From that day to the death of Joule (1889) these two 

 men were closely associated in the demonstration and exploitation of 

 a great principle of which they were at first almost the sole exponents 

 among English-speaking people. 



