CH. XXXIV. HEAT. 329 



CHAPTER XXXIV. 



SCIENCE OF THE NINETEENH CENTURY (CONTINUED). 



Early Theories about Heat — Count Rumford shows that Heat can be 

 produced by Friction — He makes Water boil by boring a Cannon — 

 Davy makes two pieces of Ice melt by Friction — His conclusion 

 about Heat— How 'Latent Heat' is explained on the theory that 

 Heat is a kind of Motion — Dr. Mayer suggests the Determination of 

 the Mechanical Equivalent of Heat — Dr. Joule's Experiments on 

 the conversion of Motion into Heat — Dr. Hirn's Experiments on the 

 conversion of Heat into Motion — Proof of the Indestructibility of 

 Force, and Conservation of Energy. 



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 ther- 

 mometer, swelled it out and made it rise. This was the 

 general idea about heat up to the end of the eighteenth 



