HEAT. 



The experiments by which this 'has 

 usually been determined, are similar to 

 those already mentioned. Equal portions 

 of the same fluid at different tempera- 

 tures being mingled together, the mean 

 temperature results ; but when two 

 bodies of different kinds are used, the 

 resulting temperature is never the mean 

 between the two ; as, when a pound of 

 water heated to 156 is mixed with one 

 pound of quicksilver at 40, the tem- 

 perature produced is 152 instead of 98, 

 the exact mean. 



In this experiment the water loses 

 four degrees of temperature, and the 

 quicksilver gains one hundred and 

 twelve degrees ; by which it is proved 

 that the quantity of caloric which is 

 capable of raising one pound of quick- 

 silver from 40 to 152 is the same as 

 that which is required to raise one 

 pound of water from 152 to 156; or, in 

 other words, that the same quantity of 

 heat which raises the temperature of 

 one pound of water four degrees, raises 

 the same weight ' of quicksilver one 

 hundred and twelve degrees. On this 

 account it is said that "the capacity of 

 water for heat is to the capacity of 

 quicksilver for the same, as 28 to 1. 



The experiment being reversed by 

 mixing one pound of quicksilver, heated 

 to 156, with one pound of water at 40, 

 the resulting temperature will be 44; 

 the water acquiring an increase of 4, 

 while the quicksilver loses 112. 



A pound of gold heated to 150 was 

 quickly added to a pound of water at 

 50, by Dr. Black; the temperature of 

 the whole became 55, the gold losing 

 95 and the water gaining 5, making 

 the capacity of the gold, compared with 

 that of the water, as 1 to 19. 



The general rule given for finding, by 

 calculation combined with experiment, 

 the relative capacities of different bodies 

 is as follows : 



" Multiply the weight of each body by 

 the number of degrees between its origi- 

 nal temperature and the common tem- 

 perature obtained by their mixture: 

 the capacities of the bodies will be in- 

 versely as the products." Or, if the 

 bodies be mingled in unequal quantities, 

 " the capacities of the bodies will be 

 reciprocally as the quantities of matter 

 multiplied into their respective changes 

 of temperature." 



If we compare the quantities of calo- 

 ric which are necessary to raise equal 

 volumes of different substances to any 

 given temperature, they will also be 



found different: water requires more 

 than twice as much caloric to raise its 

 temperature any given number of degrees, 

 as the same volume of mercury requires. 



A variety of different methods have 

 been used for the purpose of ascertain- 

 ing the capacities of bodies, by finding 

 the comparative quantities of caloric 

 which they contain at different tem- 

 peratures. The method of Wilcke, who 

 operated on solid substances, was to 

 suspend given weights of them, by 

 threads, in boiling water, until they ac- 

 quired the same temperature ; they 

 were then suspended in cold water, and 

 the quantity of heat imparted to the water 

 carefully calculated. 



It also occurred to him that the spe- 

 cific heat or comparative quantities of 

 caloric existing in bodies at given tem- 

 peratures, might be ascertained by the 

 quantities of ice or snow capable of 

 being melted by each ; but this method 

 proved unsatisfactory, chiefly on account 

 of the great difficulty in determining how 

 much of the water produced remained 

 adhering to the unmelted portions of ice 

 or snow. 



An instrument called a Calorimeter 

 was invented, and used in similar inves- 

 tigations, by Lavoisier and La Place. 

 In this instrument there are three ves- 

 sels within one another, the innermost 

 of which a, {fig' 13) is of open 

 wire-work, and is for holding the body 

 on which experiments are to be made : 

 it is figured upon a larger scale at a 2 : 

 it rests upon bars of iron, which com- 

 municate with the interior of the middle 

 vessel b : into this vessel the ice is 

 put, broken small, that it may be more 

 readily acted upon by the heat of the 

 body placed in the interior cage ; the 

 water produced passes through a grating 

 at the bottom, and is conveyed by the 

 pipe d into a vessel e placed to receive 

 it. The third vessel c is intended to 

 hold ice and water, to prevent the tem- 

 perature of the atmosphere from affect- 

 ing the experiment by surrounding the 

 ice to be operated upon by a temperature 

 of 32. The cage has its own cover f\ 

 and, in addition, the whole apparatus is 

 furnished with a double cover g, capa- 

 ble of holding pounded ice. 



The instrument being prepared, a 

 heated body, the temperature of which 

 is ascertained, is put into the cage, 

 where it remains until it is cooled to 

 32 ; the heat given out by the body is 

 then estimated by the quantity of water 

 produced. 



