QUANTITY OF HEAT. SPECIFIC HEAT. 



83 



When the test tube is held vertical, the substance, which has been 

 brought to any desired temperature, drops into the calorimeter, and some 

 of the liquid boils off. A branch tube leads from near the upper end 

 of the vertical tube, and the evolved gas, passing through it, is collected 

 in a receiver and measured. The arrangement is similar in principle to 

 V. Meyer's Vapour-Density Apparatus (Fig. 102, p. 177). A gramme 

 calory was found to evolve 13-2 c.c. of oxygen, 15-9 c.c. of nitrogen, 

 and 88 -9 c.c. of hydrogen, measured in each case at C. and 760 mm. 

 The observations were reduced by comparison with lead, of which 

 the specific heat had previously been found to increase very nearly 

 uniformly from 0-0280 at - 220-5 C. to 0-0295 at - 85 C. After an 

 experiment had been made with the substance under investigation, a 

 similar experiment was made with a quantity of lead, so chosen that 

 about the same quantity of gas was evolved. 



The specific heats of a number of substances at various ranges down 

 to 188 C. were determined, and the following results were found for 

 diamond, graphite, and ice : 



Referring to the original paper for details, we may note here that 

 Dewar was able to determine with the calorimeter the latent heats 

 of oxygen, nitrogen, and hydrogen. He also found that the specific 

 heat of liquid hydrogen is 3 -4, the value which it has in the gaseous 

 condition. 



Influence of Change of State on Specific Heat. The difference 



between the specific heats of diamond and graphite is an illustration of 

 the fact that the particular condition of a substance, crystalline or 

 amorphous, softened or hardened, affects its specific heat, though, as a 

 rule, the variation with such condition is not great. But as a substance 

 changes from the solid to the liquid or from the liquid to the gaseous 

 state, the specific heat may change very considerably. As typical, we 

 may take water and lead. We have : 



Ice. . -5 Water . 1-0 



Solid lead -0314 Molten lead -0402 



Steam (constant pressure) -34 



As a rule, the change is in the same direction for other substances, 

 but there is no known relation between the specific heats of the same 

 substance in the different conditions. 



Atomic and Molecular Heats. There is undoubtedly some 

 relation between the specific heat of a substance and its atomic or 

 molecular weight. Thus for the less condensable simple gases the product 



