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 understand 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. Still most scientific men looked upon heat as a fluid, which they called caloric, until, in the year 1798, Count Rumford first showed by experiment that it is probably a kind of motion. In following strict chronological order, this discovery ought to have been mentioned at the end of the eighteenth century, but it belongs so intimately to the modern theory that it comes more naturally in this place. Count Rumford shows that Heat can be produced by Friction, 1798.-Benjamin Thompson, afterwards Count Rumford, was born in the United States in 1753. He spent his early life fighting in the English army against the Americans, in the War of Independence, and afterwards settled at Munich, and became aide-de-camp to the Elector of Bavaria. In 1798 he came over to England, where he was one of the founders of our Royal Institution, and finally he died in Paris in 1844. Rumford's inquiries into the nature of heat began in rather a curious way. He was very anxious to make the poorer people in Bavaria happier and more prosperous, and to accomplish this he persuaded the Elector of Bavaria in 1790, to forbid anyone to beg in the streets. Those who could not find work for themselves were taken up and kept in a kind of workhouse, where they were given good food and clothing, but were forced to work to pay for their own support. When this law was first passed, there were no less than 2,500 beggars to be provided for, and Rumford was obliged to calculate very closely how he could find food and clothing, heat and light, for the least money. CH. XXXIV. COUNT RUMFORD. 331 Accordingly he studied how fire-places could best be built to prevent coal being wasted, and invented a lamp which gave a brilliant light, without burning so much oil as other lamps did. He even went so far as to make a complete set of experiments on different clothing materials, in order to see which kept in the most heat. It was in this way, and especially in using steam for warming and cooking, that he first began to study the properties of heat, and he became much interested in the different ways in which it may be produced. It happened one day, when he was boring a cannon in one of the military workshops of Munich, that he noticed with surprise the great heat produced by the grinding of the borer against the gun. You can easily make a similar experiment by boring a hole quickly with a gimlet in a piece of hard wood, and on withdrawing the gimlet you will find that it is hot enough to burn your hand. Rumford examined carefully the gun and the chips which fell from it, and found that they were both hotter than boiling water. This led him to consider how it could possibly happen, if heat were a fluid, that the mere rubbing of two metals together should produce it; and he tried many experiments to find out whether the gun, the chips, or the borer had lost anything in consequence of having given out heat. But he could not discover that they were changed in any way; and moreover, he found that by going on boring he could make them give out heat as long as he liked, whereas if he had been drawing a fluid out of the metals it seemed to him that it ought to come to an end sooner or later. Then he considered whether the heat could come out of the air, and to avoid this he repeated the experiment under water, but still the metals grew hot, and even made the water warm, so it was clear they had not drawn any heat from that fluid. He now began to suspect that Bacon and Locke might be right, and that the rubbing together of the two metals might set their particles vibrating in some peculiar way so as to cause what we call heat. If this were so, then by great friction he ought to be able to produce any amount of heat, and to prove this he tried the following experiment. He took a large piece of solid brass the shape of a cannon, and partly scooped out at one end. Into this he fitted a blunt steel borer, which pressed down upon the brass with a weight of ten thousand pounds. Then he plunged the whole into a box holding about a gallon of water, into which he put a thermometer, and fastening two horses by proper machinery to the brass cylinder he made them turn it round and round thirty-two times in a minute, so that the borer worked its way violently into the brass. Now notice what happened: When he began the water was at 60° F., but it soon grew warm with the heat caused by the friction of the borer against the brass. In one hour it had risen 47° up to 107° Fahr. ; in two hours it was at 178°, and at the end of two hours and a half it actually boiled. 'It would be difficult,' writes Rumford, 'to describe the surprise and astonishment of the bystanders on seeing so large a quantity of water heated and actually made to boil without any fire,' and he adds that he himself was as delighted as a child at the success of the experiment; and we can scarcely wonder, for he had proved the grand fact that motion can be turned into heat! Rumford afterwards calculated that the friction caused by one horse pulling round the cylinder against the borer CH. XXXIV. SIR HUMPHREY DAVY. 333 was sufficient to raise 26 lbs. of ice-cold water up to the boiling-point in two hours and a half. Davy makes Two Pieces of Ice melt by Friction in a Vacuum, 1799.-Only a few months after Rumford had made the discovery that heat can be produced by friction, Sir Humphry Davy, whose history as a chemist you will read in chapter xxxvi., proved the same thing by a different experiment. He took two pieces of ice, and by rubbing them together made them melt without any warmth being brought near them. In this case, as he said, no one could think that the heat came out of the ice, for ice holds less heat than water; and in order to be quite sure that it did not come out of the air, he made a second experiment. He took a small piece of ice and put it in a machine under an air-pump, by means of which he drew out all the air; then he set his machine to work so that it rubbed against the ice, and in this way he melted the whole lump, without any being present. air Heat a Vibration.-From these experiments Davy came to the conclusion 'that heat is a peculiar motion, probably a vibration of the corpuscles (that is the little particles) of bodies, tending to separate them.' Thus for example, when you put a saucepan full of water on the fire, the quivering motion which is going on in coals as they burn passes into the iron of the saucepan, and through it to the water. Immediately all the little particles of which the water is composed are pushed asunder as if they were trying to get away from each other; but as they are still held together by the force of attraction, they vibrate to and fro, struggling more and more to get free, and it is this motion which causes in us the feeling of heat when we come in contact with it. Then, if a thermometer be placed in the water, the vibration |