Every substance has a characteristic called specific heat. This is the measure of the temperature change in a substance when a given amount of heat is applied to it.
One Btu (British thermal unit) is the amount of heat required to raise the temperature of 1 lb of water by 1° at 39º F. With a few exceptions, such as ammonia gas and helium, all substances require less heat per pound than water to raise the temperature by 1°F.
Thus, the specific heat scale is based on water, which has a specific heat of 1.0. The specific heat of aluminum is 0.2. This means that 0.2 Btu will raise the temperature of 1 lb of aluminum by 1°F. One Btu will raise the temperature of 5 lb of aluminum by 1°F, or 1 lb of the same by 5°F.