A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

STEAM. 35 (4) In the fourth column is stated the “ total ” heat of steam—that is, the total number of heat units required to raise 1 lb. of water from 32° (freezing point) to boiling point, plus the heat required to convert that 1 lb. of water into I lb. weight of steam at different pressures. (5) In the fifth column is given the “ specific volume ” or density of steam—that is, the ratio in volume between 1 cubic foot of water and the number of cubic feet of steam that it is convertible into at the particular pressure indicated. (6) In the sixth column is given the weight (in lbs. avoirdupois) of 1 cubic foot of steam at the particular pressure. Heat—Measurement of. 4. Heat, although only the condition of a substance, is capable of measurement just as much as the substance itself is, with regard to its weight or its volume. A ther- mometer hanging in the air and recording, say 60°, on being plunged into water records, say 40°, shows that the condition of the air as regards temperature is 20° hotter than the water. But if a given weight of any substance, say 1 lb. of that water, has a certain amount of heat im- parted to it, which raised it from 39° to 40°, the 1° of heat thus absorbed, and capable of returning to some other substance, affords a means of standardising the condition of any substance as regards the heat it is capable of absorbing from or imparting to any other substance. This heat standard is called in this country a thermal unit. 5. A thermal unit, usually referred to as one B.T.U. (British Thermal Unit), is the specific heat required to