On the Production of Steam from heated Iron, 71 



Art. IX. — 'Observations and Experiments on the variable rapidity 

 of action, between ivater and hot iron; by Walter R. Johnson, 

 Professor of Mechanics and Natural Philosophy in the Franklin 

 Institute, Philadelphia. 



The several series of experiments heretofore detailed, in relation 

 to the actual quantity of vapor yielded by red hot metal, and to the 

 time employed in producing it, have furnished some of the data for 

 calculating the effect of overheating a steam boiler and immediately 

 furnishing it with water. It is evident, that even with the same tem- 

 perature in the metal, certain circumstances may exist at one time 

 which shall modify the result exhibited at another. The tenth ex- 

 periment in the fourth series,'^ in which 60 ounces of metal continued 

 red for 82 seconds, beneath the surface of boiling water, and after- 

 wards occupied 46 seconds in parting with the excess of heat above 

 212° which then remained, might possibly lead to the inference, that 

 the quantity of heat disengaged in the former part of the operation 

 was at least twice as great as that which was given out in the latter. 

 This would imply that the temperature, (omitting difference in spe- 

 cific heat,) had been at first three times as much above 212°, as it 

 was at the moment when redness disappeared. But the whole of the 

 fourth series, as well indeed as all the other series heretofore given, 

 had manifested in the performance of the experiments, a much more 

 vigorous action subsequent to the disappearance of redness, than be- 

 fore that period. It was therefore necessary, in order to obtain some 

 degree of clearness on this head, to perform several courses, each 

 consisting of a number of series of experiments. 



The general fact that red hot metal repels water, or at least does 

 not appear to exercise upon it any contiguous attraction, has long 

 been familiar. The smith who plunges a piece of iron, at a white 

 heat, into his trough, sometimes sees with astonishment that scarcely 

 any agitation of the liquid occurs for the first few seconds ; and he 

 perceives that this is not due to the coldness of the water, requiring 

 it to be heated up to boiling temperature, before it can undergo the 

 agitation consequent upon ihe formation of steam ; for by plunging 

 another piece of metal at a black heat into the same liquid, the ac- 

 tion becomes immediately and distinctly perceptible. 



When water is sprinkled upon a stone plate, even below redness, 

 the drops are often observed to roll, apparently with litde or no adhe- 

 sion, from side to side, until slowly dissipated, or until they at length 



* See Vol. XX, p. 311, of this Journal. 



