In parts of the country where the temperature goes below 60° F. it becomes necessary to provide artificial heat to warm houses in order that health may be maintained at the least possible expenditure of energy. The most common method of producing heat for heating purposes is by combustion. Its obedience to certain physical laws is infallible. Heat is liberated from such material as coal and wood by combustion and is the result of the chemical action of this combustion; it is then transmitted to the rooms to be heated either by air, water, or steam unless the combustion takes place in the room to be heated; then it is distributed throughout the room by radiation from the open fire or conduction from above. Molecular Theory Up until the beginning of the nineteenth century heat was believed to be a substance that had no weight and the name caloric was given this hypothetical substance. Davy and Rumford, through a series of experiments, proved that heat is a violent agitation of the molecules of matter. From this we have the molecular theory that as the velocity of the molecules is increased heat is produced and the temperature raised. The words heat and temperature are not interchangeable. Heat is the cause and temperature is the effect. Temperature indicates the presence of heat and the degree of temperature represents the intensity of the heat, but not the quantity. Heat in the same amount may be imparted to two bodies of the same substances, but different mass, and one will be hotter than the other; therefore, the specific heats of the two substances are different. To illustrate: Place in the sun a receptacle containing two gallons of water and one containing one gallon of water, both the same temperature. Leave them for a given length of time and they will become warm, but the one gallon will be warmer than the two, because of the difference in the amount of water to be warmed. The same amount of heat was applied to each, but this did not produce the same temperature in both. Again the same quantity of different substances may be exposed to the same heat, but the temperature will not necessarily be the same, for some substances heat more rapidly than others. It requires more heat to raise the temperature of water to a given degree than it does the same weight of any other substance, except hydrogen. This is the reason water gives off more heat than any other substance that cools through the same number of degrees. Normal heat is produced in the body by the expression of calorific mental impulses and by the oxidation which is carried on in the tissue cells. The amount of heat produced in the body is adaptative to the needs of the body and is under the direct control of Innate Intelligence.