ART. 67. _First Law of Thermodynamics._--The Law of Thermodynamics is based on two fundamental truths which have reference to the conversion of Heat into Work, and Work into Heat. In Art. 54 we have already seen that energy in the form of heat, light, electricity and magnetism is capable of being converted into other forms of energy, while in Art. 59 we have seen that Joule gave us the exact relation in foot-pounds between heat and work. He showed that when 1 lb. of water fell through 772 feet its temperature was raised one degree Fahr. Thus the principle underlying the first law of thermodynamics states, that whenever work is spent in producing heat, the amount of work done is proportionate to the quantity of heat generated; and conversely, whenever heat is employed to do work, a certain amount of heat is used up, which is the equivalent of the work done. This principle is also in accord with the conservation of Energy and Motion (Arts. 52 and 57), which assert that whenever energy or motion disappears in one form, it is manifested in some other form. Thus, from the first law of thermodynamics, we learn that wherever we have heat we have the power to do work, and the amount of work so done is proportionate to the heat used up. Heat, then, has a capacity to perform work, and that power is known as the mechanical equivalent of heat. Both Mayer of Germany, and Dr. Joule of Manchester, have worked out this problem, and have given us the mechanical value of heat. By experiments Mayer found out that a quantity of heat sufficient to raise 1 lb. of water one degree Fahr. in temperature was able to raise a weight 771.4 lb. one foot high. Dr. Joule of Manchester, after making a number of experiments which lasted over many years, came to the conclusion that the mechanical equivalent of a unit heat was 772 foot-pounds, a unit of heat being the quantity of heat which would raise 1 lb. of water one degree Fahr. So that if a 1-lb. weight fell from a height of 772 feet, an amount of heat is generated which would raise 1 lb. of water one degree Fahr.; and conversely, to lift 1 lb. 772 feet high, one degree Fahr. of heat would be consumed.