82 On the Production of Steam from heated Iron. 
of redness, numerous other facts had convinced me that the approach 
to this period is greatly accelerated by the adhesion of any non-con- 
ducting substance to the surface of the iron. Indeed, it often appear- 
ed sufficient for the water to find and seize upon a mere point of such 
material as a nucleus, to enable the fluid speedily to reduce the tem- 
perature of the surrounding surface. By detaching a scale of oxide, 
around which the effect just described had begun to take place, I have 
sometimes succeeded in arresting the progress of vapbrization, and 
by giving the liquid once more a clean red surface, even with the 
scale floating loosely in the water, to establish once more the slow 
evaporation which belongs to that state of the metal. 
To ascertain what effect the incrustation generally formed upon 
the interior of a steam boiler might be expected to produce, in aug: 
menting the rapidity of action in a case of overheating, I performed 
the following course of nine series, employing for that purpose, the 
basin used in the first course, commencing with its surface clean, an 
having tried the effect of pure water at 212°, subsequently poured 
in a portion of cold water, into a pint of which about two ounces 0 
clayey garden earth had been put, producing a degree of turbidness 
as great probably as any of our rivers possess in the time of freshets- 
The iron was kept constantly over a brisk fire, and, in some of the 
series, was permitted to come to bright redness before each expell- 
ment; while in others, the operation commenced with redness, but 
was continued in so immediate a succession, as to reduce the met 
to a certain point of constant action; but never attaining the most 
rapid period, 
It will be perceived that the first series was made in pairs, alter- 
nately—two with clean water at the boiling point and two with the mud 
dy water above mentioned. The other series were made with similar 
alternations of single experiments, with the exception that both hot 
and cold water were free from impurities when laid upon the metal. 
The ratios placed among the results of this course, will prove that 
on an average, water at 212° laid upon hot metal under the circul 
stances described, requires 154 per cent. longer for its evaporation 
than a like quantity of water at 60°. This result, which appears at 
first rather startling-and paradoxical, is readily explained when ¥° 
consider the efficacy of cold water in bringing the coating and evel 
the surface of the metal down towards the temperature of most rapi 
action,—a point, at which the mere difference of temperature be- 
comes an insignificant element in the calculation, compared wi 
vastly augmented speed with which the vapor is then generated. 
