Experimental Inquiries respecting Heat and T^apor. 305 



ced over an argand spirit lamp. These portions were not of suffi- 

 cient amount or frequency to prevent the increase of temperature in 

 the metal, and consequently the times of vaporization were diminish- 

 ed to a certain point, after which they were ohserved to increase. 

 The temperature had then reached the point where repulsion begins. 

 The temperature at the moment when the point of repulsion appear- 

 ed to have been attained was noted, and the experiments continued 

 until an unequivocal increase in the time of evaporating the unit of 

 water was observed. The lump being now withdrawn, the tempera- 

 lure was allowed to descend, and the rapidity of vaporization was of 

 course augmented ; still lowering the temperature, the point of great- 

 est action was passed, and the production of steam became slower 

 from want of sufficient heat. 



By thus reversing the temperatures, and alternately passing and 

 repassing the point of most vigorous action, the limits of that action 

 were determined to a certain degree of exactness. It soon became 

 evident, that it was far below the boiling point of mercury, and con- 

 siderably above that of water boiling in open air. It was not diffi- 

 cult to ascertain too, that the range of most rapid action lay between 

 300° and 350°. In order to vary the mode of experimenting, and, 

 at the same time, to give more exact indications in several particu- 

 lars, the second method, above referred to, was devised. This con- 

 sisted in employing a bar of iron, about 14 inches long, ly\ wide, 

 and ly\ thick. A number of cylindrical holes, half an inch in diam- 

 eter, and one inch apart, (from centre to centre,) were bored along 

 one of the sides, extending nearly through the thickness of the bar. 

 Adjacent to each of these holes, which were five in number, were 

 sunk small conical cavities, j\ of an inch deep and /^ of ^^ inch in 

 diameter at top, forming basins or cups to receive drops or other 

 small measured portions of liquids. The cylindrical holes were to 

 receive mercury, into which the bulbs of thermometers could be 

 plunged, to ascertain the temperature of the part of the bar and of 

 the cup opposite. The thermometers were supported from above, 

 by hooks bent over the bar and placed in proper positions to allow 

 the bulbs to descend just far enough to be completely immersed in 

 the reservoir of mercury, but not to carry the centre of the bulb 

 below the level of the bottom of the contiguous cup. 



By this means the temperature of the mercury was measured, at 

 a point where it must have been the same as that of the generating 

 surface. The five receptacles of mercury were placed near the 



Vol. XXL— No. 2. 39 



