Experimental, Inquiries respecting Heat and Vapor. 311 



to give a maximum energy of action between it and the spirit. By 

 several trials for this purpose, it was found to be three and a half 

 seconds. The greatest length of time during, which the same quan- 

 tity had been found to remain was one hundred and fifty seconds. 

 Whence it appears, that the relation between the two is fi\='sloi 

 or j\ nearly. The only remaining question was the actual temper- 

 ature at which the spirit disappeared in the least time. For this pur- 

 pose, recourse was had to the bar with mercurial reservoirs and cups, 

 already described. On raising the temperature to 312°, where wa- 

 ter had been observed to be most rapidly vaporized, it was manifest 

 that the alcohol was clearly and strongly repelled. 



The temperature was then lowered to 280°, when occasional signs 

 of adhesion were manifested, and a corresponding diminution in the 

 time of evaporating a given quantity of liquid was the result. 



By lowering the temperature of the iron to 260°, the time was 

 again perceived to increase on account of a deficiency of heat. By 

 thus passing and repassing several times between 260° and 280°, 

 the limits of range became circumscribed between 270° and 278°, 

 and finally the point of most vigorous action seemed to rest at 274°, 

 the arithmetical mean of the above mentioned limits. This, it will 

 be recollected, is 100° above the boiling point of alcohol. It will 

 be observed also, that this is exactly as much above its boiling point, 

 as the temperature of most activity on water is above the boiling point 

 of that liquid. 



6. An allusion has already been made to the opinion of some writers, 

 that the repulsion of a liquid from metal begins at the temperature of 

 incandescence, and increases as the temperature rises. The facts 

 already detailed in this paper, will serve to show that the former opin- 

 ion is wholly without foundation. Indeed, when we reflect for a mo- 

 ment on the nature and cause of that diminution of the liquid which 

 takes place after vaporization has ceased through an excess of tem- 

 perature, we must perceive that as the effect is an evaporation, due 

 to the radiation of heat, the rapidity with which the latter will dis- 

 perse a given quantity of water must be proportionate to the tension 

 of the heat at the radiating source ; that is, the surface of the metal. 

 Evaporation must commence where vaporization ceases, and the 

 former must be slow when the tension is barely sufficient to elevate 

 the liquid out of the sphere of contact, or of contiguous attraction. 

 This cannot however prevent an increase of rapidity, when the ten- 

 sion at the source is sufficiently elevated to allow the radiated heat to 

 communicate temperature to a transparent medium. 



