CH. xxxv. THEORIES OF HE A T. 349 



CHAPTER XXXV. 



SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). 



Early Theories about Heat Count Rumford's Experiments Davy's 

 Experiments Mayer Dr. Joule on Mechanical Equivalent of Heat 

 Indestructibility of Force, and Conservation of Energy Dissipa- 

 tion of Energy Molecular Theory of Gases Free Molecules ia 

 Vacuum Tubes. 



Early Theories about Heat. From Light we will now 

 pass on to Heat, and in this chapter I hope to show you how 

 the philosophers of this century have discovered what heat 

 is. The subject in itself is so vast that a mere sketch of all 

 the men who have worked at it and their chief experiments 

 would fill a volume of this size, and you must clearly under- 

 stand that we can only select those examples which will best 

 enable you to comprehend the nature of heat, and how it 

 has been determined. 



Have you ever asked yourself what heat is, or why the 

 mercury in a thermometer rises when it is put into hot 

 water? The old philosophers considered heat to be a fluid, 

 which passed out of substances when they were too full of 

 it, and which, entering the mercury of the thermometer, 

 swelled it out and made it rise. This was the general idea 

 about heat up to the end of the eighteenth century, although 

 Lord Bacon, more than two hundred years before, had 

 suggested that it was not a fluid but a movement, and the 

 philosopher Locke, in the seventeenth century, and Laplace 

 in 1780, gave the same explanation. 



