306 Experimental Inquiries respecting Heat and Vapor, 



middle part of the bar, leaving a part four and a half or five inches 

 long at each end, without holes ; but the line of cups already men- 

 tioned was extended in both directions, nearly to the extremities of 

 the bar. By this means the nature and mode of action could be 

 observed, at points above that of mercurial ebullition. 



Heat was applied at one end of the bar, either by means of a 

 spirit lamp, or by thrusting the end into an opening through the side 

 of a furnace. As the temperature rose, the cups near the end next 

 the fire, were, of course, first brought to a vaporizing temperature ; 

 then the cup opposite to the nearest mercurial reservoir and the 

 others in succession, with greater or less rapidity according to the 

 tension of the heat at its source. It was generally found most ad- 

 vantageous to employ, for a source of heat, the convenient chemical 

 spirit lamp with argand burner, which has been devised by Dr. J. K. 

 Mitchell. When the temperature was sufficiently raised, drops of 

 water were simultaneously projected into two or more of the cups, 

 and by the inequality in the times of final disappearance, their rela- 

 tive influence was easily perceptible. This mode of operating, by 

 allowing the temperature to be gradually raised admitted of a suc- 

 cession of five series of trials, one for each cup, so that when the 

 time of vaporization, in one, had begun to increase, that is, when the 

 time of most rapid action in that cup had been passed, and the action 

 had become slow through excess of heat, it was only necessary to 

 commence with the next cup, more remote from the source of heat. 

 The period of greatest rapidity was now perceived to lie between 

 304° and 320°. The range of temperature through which the most 

 rapid action existed was hence limited between two points, equally 

 remote from 312°, or from 100° above the boiling point of water. 



2. The nature of the effect here observed resembled that of vig- 

 orous attraction. This necessarily creates a constant struggle be- 

 tween the vapor which is quitting, and the liquid which is approach- 

 ing any given point of the metallic surface. On brass, the action 

 appeared more vigorous, and the temperature of repulsion higher 

 than in the case of iron. On mercury, at 600°, a drop of water 

 was, on one occasion, found to remain seventy seconds ; but at 340° 

 a drop of this metal formed a good nucleus, about which the water 

 when repelled by a surface of iron, at the same temperature, would 

 gather, and thence obtain heat to vaporize itself, while portions not 

 in contact with the mercury would lie upon the iron almost quiescent. 



