HEAT 573 the same law as for solids, but is much more feeble, the conductivity of water being only about V that f copper. Liquids are readily heated by convection. When heat is applied beneath vessels containing them, the stratum next the bottom expands by heat, and in rising the particles communicate their excess of heat to those through which they pass. Gases become heated in the same manner ; they are exceedingly bad con- ductors, but from the mobility of their particles it is difficult to arrive at satisfactory results as to their conducting power. Po- rous substances containing con- fined air are bad conductors of heat, wherefore the walls of well built dwellings intended to ex- clude the heat of summer and the cold of winter are divided into partitions containing con- fined air. Plaster of Paris, on account of its porosity after set- ting with water, and its non- combustibility, is used for filling between the plates of fire-proof safes ; and the efficiency of po- rous garments in protecting the body against cold and heat is a matter of common observation. FIG. 8. There is a remarkable exception to the non-conductivity of gases in the case of hydrogen, which, although the .ightest of all of them, is by comparison far the best conductor of heat. This is proved by the following experiment : If a fine platinum wire is passed through a glass tube, as shown in fig. 8, and its two ends placed in connection with the poles of a galvanic battery, it will be- come incandescent on the passage of a moderate galvanic current, if air or any gas besides hy- drogen is passed through the tube, though not to the same degree as in a vacuum ; but if hydrogen gas is passed through the tube, the incandes- cence disappears in consequence of the heat being conducted away. III. SPECIFIC HEAT. The first important experiments upon the spe- cific heat of bodies were made by Dr. Black in the latter part of the 18th century, and the idea of measuring specific heat originated with him. If two equal measures of water are placed in separate vessels of the same material, all being at the same temperature, and there is immersed in one an iron ball of a certain weight, and of a temperature higher than that of the water, and in the other a quantity of mercury of equal weight and temperature, after a time each of the vessels with their contents will have come to an equilibrium ; but it will be found that the contents of the vessel in which the iron was placed have a higher temperature than the other, showing that the iron has com- municated to the water a greater quantity of heat than the mercury. If the iron ball and the mercury had been colder than the water, on the attainment of equilibrium the water con- taining the iron would have been colder than that which contained the mercury. The amount which a body is thus capable of imparting or absorbing while rising or falling through a cer- tain range of temperature is called its specific heat. The term first used to denote this prop- erty was " capacity for heat," and was intro- duced by Irvine, a pupil of Dr. Black. The term specific heat, according to Whewell, was proposed by Wilcke, a Swedish chemist, and according to others by Gadolin, of Abo, in 1784. If, in the experiment just mentioned, instead of an iron ball, an equal weight of water at the same temperature had been used, the quantity of heat imparted to the water already in the vessel would have been very much greater. If equal w eights of water at different temperatures are mingled, the result- ing temperature will be a mean between the two ; but when equal weights of iron and water at different temperatures are placed together, the resulting temperature will be nearer that of the water. In making experiments in spe- cific heat, it is necessary to adopt some unit of measure, of which several are employed. The gramme degree (centigrade) is the quan- tity of heat required to raise one gramme of water 1 C. ; the kilogramme degree, some- times called a calorie, is the heat required to raise one kilogramme of water 1 C. ; and the pound degree is the amount required to raise one pound avoirdupois of water 1 F. or C. Three methods have been employed for deter- mining specific heat: 1, the method of fusion of ice ; 2, the method of mixtures ; and 3, the method of cooling. 1. The method of fusion of ice. This was employed by Black, and simply consisted in making a deep cavity in a block of ice, fig. 9, placing the substance to be experimented on in it, and closing the cavity with a cover of ice. The substance is raised to a certain temperature, then introduced, and when cooled to zero is removed, and both it and the cavity wiped with a cloth of known weight ; the increase in weight shows how much of the ice has been melted. Now, as will be seen further on, it re- quires as much heat to convert a pound of ice at 32 to a pound of water at 32, as it does to raise a pound of wa- ter from 32 to 174 ; therefore water at 32 contains 142 more heat than ice at the same temperature. Let m denote the weight of wa- ter derived from the ice in the above experi- ment, w the weight of the body under experi- ment, s its specific heat, and t the number of degrees it has fallen ; then there will re- sult the following equation: w t sl^m or S _142? from wll i c h formula the specific heat FIG. 9. Black's Ice-block Calorimeter.
