If you take hold of an iron rod that has just been removed from the fire, it feels hot; on the other hand, if you touch a piece of ice, it feels cold. The cause of these sensations is said to be heat. The warmer body always gives off heat to a colder. For many years it was believed that heat was a fluid, called "caloric;" about the middle of the nineteenth century the experiments of Joule proved that a definite amount of mechanical work is equivalent to a definite form of heat. In other words, heat is a form of energy. Heating or cooling is merely a transformation of temperature or "heat level," as before we might have spoken of a higher water level. To measure temperature the simplest instrument in use is the thermometer, a long glass tube on one end of which is a bulb partly filled with mercury. The tube is open at the upper end after the mercury is poured in. The bulb is then heated till the mercury rises to the top, when the tube is sealed by means of a blow-pipe. As the bulb cools the mercury recedes, leaving a vacuum at the upper end of the tube. It is clear that there must be some point at which all thermometers agree. Careful investigations have made it certain that under uniform conditions the temperature of melting ice and that of steam are invariable. These points are generally known as freezing point and boiling point. On the centigrade scales (centigrade from centum, meaning hundred, and gradus, meaning steps) the freezing point is marked 0deg. Centigrade and the boiling point 100deg. On the Fahrenheit scale (named after the German scientist Fahrenheit) the freezing point is 32deg. and the boiling point 212deg. Most household thermometers are marked in the Fahrenheit scale, but for scientific purposes the Centigrade scale is much better, because it is readily reduced to decimals. It frequently happens that we are called on to change temperature readings from the Centigrade scale to the Fahrenheit, or from Fahrenheit to Centigrade. We know that 100 deg.C. equal 212 deg.-32 deg. or 180 deg.F. (the abbreviations C. and F. are commonly used instead of writing out the words). We are asked to find the equivalent in Fahrenheit degrees of a reading of 60 deg.C. We know that 100 deg. C.=180 deg. F. Then 1 deg. C.=1.8 deg.F Therefore, 60 deg. C.=108 deg. F. In other words, 60 deg. C. above zero will equal 108 deg. F. above the freezing point, because that is the Centigrade zero. But the Centigrade zero is 32 on the Fahrenheit scale. So we must add 32 degrees to give the true reading above the Fahrenheit zero. Therefore, 60 deg. C.=108 deg. F.+32 deg. F.=140 deg. F. To change a reading from the Fahrenheit scale to the Centigrade scale is just as simple; we reverse the process. To change 40 deg. F. to Centigrade degrees, we first subtract 32 deg. F. in order to find how many Fahrenheit degrees above the freezing point remain to be changed to Centigrade units. In this case we find there are 8 deg. F. above the freezing point. We already know that 180 deg. F.=100 deg. C. Then 1 deg. F.=5/9 or 0.555 deg. C. Therefore, 8 deg. F.=4.44 deg. C. It is possible that a reading above zero on the Fahrenheit scale will be below zero on the Centigrade. In this case our answer would be in minus degrees Centigrade, in other words, below zero. For the purpose of measuring the quantity of heat gained or lost by a body when its temperature changes, it was necessary to adopt a unit of heat. The one commonly used in connection with the metric system is the quantity of heat that will raise the temperature of one gramme of water one degree Centigrade. It is called a calorie. The number of degrees required to raise the temperature of a body through one degree Centigrade is the thermal capacity (from the Greek word thermos, which means heat) of the body. The thermal capacity of a unit mass of a substance is its specific heat. Specific heat bears the same relation to a calorie as specific gravity does to g or gravity, which we have already studied. For example, the specific heat of mercury is 0.033; this means that the heat which will raise 1 gramme of mercury through 1 Centigrade will raise 1 gramme of water through only 0.033 Centigrade. When a body changes from the solid to the liquid state through the application of heat it is said to melt or fuse. Freezing or solidification occurs when the body changes from the liquid to the solid state. An interesting experiment may be performed with water; if undisturbed it may be cooled a number of degrees below 0 Centigrade, but if it is disturbed it usually freezes at once and its temperature rises to the freezing point. Some substances, like wax and glass, have no sharply defined melting point. They first soften and then pass more or less slowly into the condition of a thick sticky fluid. Most substances occupy a larger volume in the liquid state than in the solid. A few substances, including water, expand when they become solids. When water freezes its volume increases nine per cent-that is the reason water pipes often burst in winter. When a body passes slowly from one state to another, there is no rise or fall in temperature. When a solid fuses, a quantity of heat disappears; and, conversely, when the liquid solidifies, an equal amount of heat is generated as was before lost. The heat required to melt one gramme of a substance without a change of temperature is called the heat of fusion. Of course, we understand that when we speak of the heat of fusion of ice as 80 calories, that we are referring to an absolute unit, merely a convenient method of measuring. No doubt you have often noticed the "sweating" of pitchers of ice water, or the dew on grass and flowers, but have you ever tried to explain these facts? You have probably said that it was cool last night and the "dew fell." The explanation is simple; the plants give up their heat very quickly after the sun sets and the moisture of the air then condenses on the cooler surfaces of the plants. Perhaps even more typical is the "sweating" on the water pitcher. This occurs in the same way. The cold pitcher gives the moisture in the warmer air a chance to condense. The word "condense" is new to our study, but surely most of us know what it means. When steam changes to water we say it condenses. Condensation is the change from vapor or gas to liquid; evaporation is the change from liquid to vapor. Like the heat of fusion, or the amount of heat required to change a solid to a liquid, there is a heat of vaporization, or the amount required to change a liquid into a vapor. This heat of vaporization is 536 calories. In other words, this amount of heat is lost before boiling water evaporates. But we have already said that no energy is ever lost, and is not heat a form of energy? So what becomes of this heat? It remains in the water as energy, which will later have the power to do work. One of the simplest illustrations of the use of steam to do work is the steam engine (see Steam Engine in Volume V and The Boy's Workshop, in this volume). Let us try to trace this energy. The great source of energy is the sun. The sun's rays give life to trees, which are cut into logs, which feed the fires, which heat the water into steam, which runs the steam engine, which operates a saw-mill, etc. We could go on indefinitely, only to find that the energy is merely transferred. For a time it may not be in use, but it still has the power when it is needed. There are three ways of transmitting heat. Place one end of a metal rod in a gas flame and the other end in melting ice. It will be found that heat passes along the rod and melts the ice. Hold your hand above the flame; it will be warmed by a rising current of hot air. Hold the hand by the side of the flame; again there will be a sensation of heat. This simple experiment illustrates the three ways in which heat may be transmitted from one point to another. The first is conduction, in which the matter conducts the heat without any visible motion of the matter itself. It is passed on from the hotter to the colder particles. The second method is called convection, which means to carry. A current of hot air or a flow of hot water through pipes is a visible transfer of heat; this is convection. The third method is radiation, by which heat is transmitted like light, by a wave motion in the ether. "Radiation" means simply that the heat travels in rays from a center; it is in this way that heat and light reach us from the sun.