HEAT AND CHANGES OF STATE. 351 



has been considered from quite a similar point of view by 

 Holtzmann in his important memoir " On the Heat and Elas- 

 ticity of Gases and Vapours." Starting from the principle 

 that elevation of temperature is equivalent to the raising of a 

 weight, this philosopher has likewise calculated the mechani- 

 cal equivalent of heat from the quantity of heat which be- 

 comes latent by the expansion of a gas ; and he very rightly 

 conceives of the latent heat of steam as made up of two 

 parts, whereof one, the smaller, is expended in overcoming 

 the opposing pressure of the atmosphere, and can hence be 

 easily calculated by means of the mechanical equivalent of 

 heat, while the remaining part, the amount of which can also 

 be calculated, is what Holtzmann calls the heat required to 

 destroy the cohesion of the water. In all steam-engines this 

 latter portion is wasted, and Holtzmann calculates from these 

 data the superior efficiency of high-pressure compared with 

 low-pressure engines.* 



If the view here taken of the latent heat of fusion and 

 evaporation is correct, heat must also become latent when 

 hard bodies are reduced to powder ; and when such substances 

 pass into the liquid condition from a state of fine division, 

 they must absorb a smaller quantity of heat than when they 

 are liquefied without previous comminution. A few experi- 

 ments that I have instituted in this direction have not hitherto 

 given any decisive result. 



It is also worthy of notice that certain solid bodies which 

 are capable of assuming allotropic states, as, for instance, the 

 oxygen-compounds of iron, evolve a considerable quantity of 

 heat on passing from a less to a more hard condition. Such 

 facts, the number of which will doubtless continually increase 

 with time, agree perfectly with the above principle, that dim- 

 inution of cohesion involves an expenditure of heat, and, on 

 the other hand, increase of cohesion a production of heat. 



* The engines which give the greatest useful effect must be those in 

 which the steam receives an addition of heat during its expansion. 



