VI. CALORIMETRIC MEASUREMENTS 181 



heat melted 10 g. of ice. This heat is, so to speak, stored in the mix- 

 ture; it can become manifest again when, for example, 100 g. of 

 steel at an initial temperature of —80° is added to the mixture. 

 Then 10 g. of water will freeze and raise the temperature of the steel 

 from —80 to 0°. Between being lost by the warmer water and fi- 

 nally gained by the cold steel the 800 cal. of heat have passed through 

 a hidden state, as it were. This state is termed "latent." One dis- 

 tinguishes thus between "sensible heat," which is observable as a 

 temperature change, and "latent heat," which produces no change in 

 temperature but instead is involved in changes of state — solid to liquid 

 or liquid to gas. 



The latent heat is used extensively in calorimetry. Famous ex- 

 amples are the ice calorimeters of Lavoisier-Laplace and of Bunsen. 



5. Chemical Energy 



The latent heat of change of state is mainly a matter of the spatial 

 relations between molecules. A further form of latent heat is in- 

 volved in chemical changes, that is, changes in the arrangements of 

 atoms in the molecules. When 1 g. of glucose is oxidized to 1.47 g. 

 of carbon dioxide gas and 0.60 g. of liquid water, 3.74 kcal. of heat is 

 produced. This is the "heat of reaction," or in this particular case 

 the "heat of combustion." For given initial and final states of the 

 process a definite amount of heat is developed from the oxidation of a 

 given amount of glucose. This amount remains the same whether the 

 process takes place as an explosion in a calorimetric bomb or as a series 

 of enzymic processes such as occur in animal tissues. The generaliza- 

 tion of this example is known as the fundamental law of thermochemis- 

 try, the law of constant heat sums, or the law of Hess, who announced 

 it in 1840. 



Three years later Robert Mayer formulated an even broader gen- 

 eralization including the relation of heat and work. As, in some mix- 

 ing trials, sensible heat, which apparently disappeared, may be con- 

 served as latent heat of state, so can mechanical work of motion be 

 conserved as potential work of position, for example in a swinging 

 pendulum. When, how^ever, the pendulum is stopped at its lowest 

 position, its work of motion as well as its potential work of position 

 have actually been lost. This loss of work is accompanied by the 

 production of heat. Work of motion can thus become latent not 

 only as work of position but also as heat ; conversely heat can become 



