1821.] Mr. Herapatk on TrireTemperahtre; %c. 363 



Article VI. 



Tables of Temperature, and a Mathematical Development of the 

 Causes and Laics of the Phenomena which have been adduced 

 in Support of the Hypotheses of " Calorific Capacity, Latent 

 Heat," &)C. By John Herapath, Esq. 



(Continued from p. 274.) 



Theory of Evaporation. 



If the parts of the particles of a fluid have a degree of adap- 

 tation which will not allow them to decompose but beyond a 

 certain temperature, the body, as soon as it has attained this 

 temperature, begins to experience a decomposition in some of 

 its particles. This decomposition must always take place in the 

 superficial particles, because, from what I have already shown, it 

 cannot in the interior, so as to produce an evaporation from that 

 decomposition, unless at the term of ebullition. As soon as this 

 decomposition takes place, the temperature in the neighbourhood 

 falls, and the parts of the fluid in this place becoming specifi- 

 cally heavier, sink. A current by this means generally ensues, 

 the cooled parts descending, and the warm rising. The higher 

 the temperature, the more rapid the currents, and the greater the 

 evaporation will evidently be. At length when the body reaches 

 the temperature of ebullition, the rapidity of the currents and of 

 the superficial decomposition are not sufficient to counterbalance 

 the rapid accessions of temperature the body is receiving, and 

 hence ebullition, or a violent decomposition in the interior of the 

 body, ensues. Pressure, as I have shown, will influence the 

 temperature at which ebullition takes place; but the pressure of 

 gases, it will appear in a future proposition, has nothing to do with 

 the absolute celerity of evaporation. Even the vertical currents 

 I have been speaking of, though in high temperatures near the 

 terms of ebullition they will always exist, are not essential to 

 evaporation. In low temperatures where the evaporation is 

 small, simple communication of temperature from the under 

 strata of the fluid may be sufficient to keep up the temperature of 

 decomposition without any current ; and hence there may be a 



erpetual evaporation even from solids, as ice, &c. which we 



now to be the case by experience. 

 Were fluids composed of particles of uniform adaptation, but 

 different in different fluids, their evaporations by this theory 

 would have all the same law and relation to their respective tem- 

 peratures of ebullition under equal compressions, which experi- 

 ments seem to confirm. But at present my object is to treat of 

 the laws and phenomena of the evaporation of water only, which 



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