I can buy five pounds of tea from my grocer or a quart of milk from the dairy; but suppose I go to my coal dealer and ask him for one ton of heat? He would probably look me over, then look at his neighbor and point to his forehead. The coal dealer can give me one ton of coal which when burnt gives heat, but neither he nor any other man can sell me one ton of heat as a market commodity.

Why not? Simply because you cannot isolate the heat and keep it. The water may be hot; the iron may be hot; many things may be; but the heat in all cases is associated with something you can see or touch. You cannot see heat and you cannot touch it. If water is hot, what you see is the water and what you touch is water. If boiling water burns your fingers whereas cold water does not, our scientist friends inform us that in reality the difference between the two states is that in the hot water, the molecules are in more rapid motion than in the cold water. The molecules are in more rapid motion. They run faster. They seem to have more energy.

Heat indeed is now known to be a form of energy, just as light and electricity are forms of energy. But if so, how are we going to measure this heat? What standard of reference can we adopt by means of which this heat can be measured? In English-speaking countries the yard and the pound are used as standards for measure and weight respectively. What standard of reference can we apply to that which we cannot weigh and cannot measure?

Water is the best-known and the most useful liquid. Suppose we take some water and heat it, and while heating it let us thrust a thermometer into the water. The mercury column of the thermometer will rise, and whenever this mercury rises we say the temperature is increasing. There seems to be a very simple relationship between the amount of heat the water acquires and the rise of mercury in the thermometer. Why should it not then be possible to measure heat in terms of the rise of the mercury column?

Calorie

That is exactly what is done. We take as a standard of reference that amount of heat which is required to raise one kilogram of water one degree centigrade, and we call this the calorie. (The kilogram is approximately equal to about two and one-quarter pounds. The kilogram, based on the metric system, is a standard of weight invariably used in scientific work and very extensively used on the continent of Europe. Since the centigrade scale of measuring temperature is based on the metric system, it is used in scientific work. One hundred degrees on the centigrade scale are the equivalent of 180 degrees on the Fahrenheit scale. The doctor's thermometer reads degrees Fahrenheit. When he says that your temperature is normal, and that it therefore is around 98, he means 98 degrees Fahrenheit. On the centigrade scale this temperature would correspond to about 37.)

Calorimeter

Now suppose we take a piece of coal, crush it and weigh one gram of it (which represents the one-thousandth part of a kilogram), and then put this one gram of coal into a vessel surrounded by another vessel containing one kilogram of water, into which a thermometer is inserted. Let us further suppose that when we burn this coal none of the heat evolved can escape except by way of the water. The heat the coal evolves in burning will therefore be transmitted to the water, and this transmission of heat will be registered by the thermometer in the water. This thermometer, let us say, will register an increase of seven degrees. The amount of heat evolved by one gram of coal when burnt will therefore be the equivalent of that produced when one kilogram of water is heated seven degrees; or it is the equivalent of seven calories. For remember that one calorie represents that amount of heat necessary to raise one kilogram of water one degree, and two calories would represent that amount of heat necessary to raise one kilogram of water two degrees; and so on. On the other hand, instead of having the weight fixed and the temperature changing, we can reverse the order and have the temperature fixed and the weight changing. For example, one kilogram of water raised seven degrees is the equivalent of seven kilograms of water raised one degree; both are equivalent to seven calories.

The calorie is the unit of heat. An arrangement for measuring heat is known as a calorimeter.*

* The calorie discussed above represents the large calorie. The small calorie is that amount of heat required to raise one gram of water one degree; it is therefore the one-thousandth part of a large calorie.