168 Mr. Ivory on the Combination of Heat 



the fi'eezing and boiling points divided into about 70 equal 

 parts or degrees, the rise or fall of the mercury on this scale 

 will show the latent heat of a mass of air varying its volume 

 under a constant pressure, at the same time that the usual 

 scale marks the temperatux'e. 



In what goes before, our attention has been occupied ex- 

 clusively with atmospheric air ; but it will readily appear that 

 the conclusions obtained extend to all the gases. For it may 

 be shown by the same reasoning as in the case of air, that when 

 a gas expands under a constant pressure, the whole heat it 

 acquires, is resolvable into latent heat and heat of tempera- 

 ture ; and that these parts are distinct from, and independent 

 of, one another. It is also a principle that holds good as far 

 as our experiments enable us to judge, that, for equal rises of 

 temperature, all the gases expand at the same rate as air. If 

 now we compare two thermometers, one of air and one of a 

 gas, the whole heat acquired by each fluid, in any given di- 

 latation, will be the same ; the heats of temperature will like- 

 wise be the same : consequently the latent heats must also be 

 the same. We are therefore to conclude, that when a gas 

 expands under a constant pressure, the whole heat acquired in 

 any dilatation, the heat of temperature, and the latent heat, 

 are to one another as 11, 8, 3, or probably more nearly, as 

 7, 5, 2. 



The theory which we have been explaining suggests some 

 reflections concerning the agency of heat. When it expands 

 air or a gas, it raises the temperature, and it enlarges the 

 volume without affecting the thermometer. These effects are 

 independent of one another ; for they may be exhibited sepa- 

 rately, and either of them may be carried to any extent while 

 the other remains unchanged. The latent heat enters the air 

 and unites with it in a manner not perceptible to our senses, 

 and increases the bulk ; the heat of temperature augments the 

 elasticitj' and affects our senses. Does heat operate according 

 to these laws only in the case of air and the gases ? or, rather, 

 is it not our power over the pressure, by which we can dilate 

 or contract a given mass of elastic fluid as we please, that 

 has enabled us to investigate the effects in question ? When 

 heat is applied to a solid or a fluid, its expansive force acts 

 against the cohesion, over which we have no control. We 

 cannot expand either of these kinds of body, and at the same 

 time keep the temperature constant; neither can we raise the 

 temperature, and at the same time keep the bulk unchanged. 

 The mode of investigation that has been pursued in air and 

 the gases, becomes impossible in solid and fluid bodies ; but 

 this does not prove that heat may not operate exactly alike in 



both 



