CALORIMETRY (Cheat measurement'), the art of measuring the quantity of heat that a body absorbs or emits when it passes from one temperature to another, or when it undergoes some definite change of state. In order to exe cute such measurements it is first necessary to adopt some convenient and accurate unit, in terms of which the quantities of heat that are to be measured can be expressed. Several such units have been proposed, but none has yet met with universal favor among physicists. One of the simplest that has been suggested (at least so far as the principles involved are concerned) is the quantity of heat that is required to melt a kilogram or a pound of ice. Evidently it will require precisely 10 times as much heat to melt 10 pounds of ice as to melt one pound and hence, if the quantity of heat required to melt one ppund of ice is taken as the unit of heat, the measurement of any given quantity of heat becomes reduced to the simple operation of observing how many pounds of ice the proposed quantity of heat can melt. The earliest form of heat-measuring device (or °calorimeter))) based upon this idea is that invented by Dr. Joseph Black about the year 1760. It consists simply of a block of clear ice, in which a is made, the cavity being closed by a slab of ice laid upon the main block. To make the use of this device plain, let us suppose that it is de sired to determine the quantity of heat that is given out by a certain fragment of platinum in cooling from 100° F. to the freezing-point. The chamber in the block of ice is first carefully wiped dry, and the platinum, heated accurately to 100°, is quickly introduced, and the covering i lid of ice is laid in place. The platinum gives up its heat to the ice about it with the result that a certain weight of the ice is melted, and a corresponding weight of water collects within the chamber. When it is certain that the platinum has attained the temperature of the ice, the slab covering the excavation in the main block is lifted off, and the water that has col lected about the platinum is removed and weighed. The quantity of heat given out by the platinum is then known at once, if the accepted unit of heat is the quantity required to melt one pound of ice. Lavoisier and La place improved Black's calorimeter in certain respects, while retaining its main features. Their instrument consists essentially of three distinct concentric chambers. The object upon which the experiment is to be performed is placed in the inner chamber, and the ice whose melting is to serve as a measure of the heat given out is placed, in the form of broken lumps, in the intermediate chamber, surrounding the object to be investigated. In the outer chamber, which
encloses the other two as completely as possible, broken ice is also introduced, to prevent the conduction of heat into the apparatus from the outside. The quantity of ice melted is deter mined by observing the amount of water that is formed in the middle chamber, this being drawn off by a conveniently situated tube and tap. This apparatus has been described as an improvement upon that of Black ; but the only way in which it can be said to be an improve ment is in the respect that it does not call for large blocks of pure clear ice. In other particu lars it is somewhat inferior to the simpler ap paratus of Black. The quantity of water that is produced, for example, cannot be determined with the same degree of accuracy in Lavoisier and Laplace's instrument. The ice calorimeter of Bunsen was a far greater advance. This in genious apparatus consists of an inner chamber, for the reception of the object to be studied, and an outer enveloping one, which is entirely filled with a mixture of Ice and water, and from which a graduated capillary tube is led away. The whole instrument is surrounded by broken ice, as in Lavoisier and Laplace's form, in order to protect the interior parts from the effect of external thermal influences. When the apparatus is in perfect working order, the mix ture of ice and water in the intermediate cham ber should be neither melted nor freezing, but should be in exact equilibrium in this respect. Upon the introduction of the object to be stud ied into the central chamber, the ice in the inter mediate chamber begins to melt, just as in the types of calorimeter already considered; but the essential peculiarity of Bunsen's instrument con sists in deducing the quantity of ice that is melted by observing the change of volume of the contents of the intermediate chamber, as shown by the motion of the water in the gradu ated capillary tube that leads away from that chamber; advantage being taken, for this pur pose, of the known fact that ice diminishes in volume upon melting, so that when the exact diminution in the volume of the contents of the intermediate chamber is known, we can calcu late with a considerable degree of precision the quantity of ice that has been melted. Bunsen's calorimeter is an admirable instrument, capable of giving results of great accuracy when intelli gently handled.