CRITICAL NOTICBS OF NEW PUBIilCATIONS. 323 



heat by melted tin, a worse conductor, and having a lower specific heat than 

 copper itself, the time of vaporization, in a spherical bowl, of quantities vary- 

 ing from l-16th to ^ of the entire capacity of the bowl, increased but three- 

 fold, and the temperature of greatest evaporation wds raised but 56**, or from 

 470** to 526**. When the bowl had half of the portion which was exposed to 

 heat filled, the weight of the water was about one and one-tenth ot that of 

 the metal. 



'•4th. The times of vaporization of different quantities of water, varying 

 from l-16th of an ounce to 2 ounces, in an iron bowl^th of an inch thick, and 

 supplied with heat by the tin bath, were sensibly, as the square roots of the 

 quantities, at the temperatures of maximum vaporization for each quantity. 



" These temperatures were raised from about 460^ to 600°, by increasing 

 the weight of water about sixteen times, indicating that considerable Quanti- 

 ties of water, thrown uDon heated metal, will be most rapidly vaporizea when 

 the metal is at least 200** below a red heat. 



"5th. While a red heat, visible in daylight, given to a metal, even when 

 very thick, and supplied by heat from a glowing charcoal fire, does not pre- 

 vent water, when thrown in considerable quantities, from cooling it down so 

 as to vaporize the water very rapidly, it is much above the temperature at 

 which the water thrown upon the metal will be most rapidly evaporated. 

 Thus one ounce of water was vaporized in 13 seconds, at about 550**, in a 

 wrought iron bowl ^ of an inch thick, and required 115 seconds to vaporize 

 in a cast iron bowl 4 an inch thick, at a red heat. Four ounces in the latter 

 bowl vaporized in about 300 seconds, the bowl being red hot when it was 

 introduced; and two ounces vaporized in 34 seconds at 600** Fah. 



"6th. The temperature of greatest vaporization, with a given thickness 

 of metal, is lower in copper than in iron, the repulsive force being develop- 

 ed at a lower temperature. With equal thicknesses of iron and copper, the va- 

 porizing power of the latter metal, at its maximum, was, with the oil bath, 

 one third greater than that of the former, and with the tin bath the power of 

 copper .07 of an inch thick, was equal, nearly, to that of iron, i of an inch 

 thick, each being taken at its maximum of vaporization, for the different 

 quantities of fluid employed. As the maxima for the iron are higher than 

 tnose for the copper, the advantage will be still greater in favour of copper 

 when the two metals are at equal temperatures. 



"7th. The general effect of roughness of surface is to raise the tempera- 

 ture at which the maximum vaporization occurs, and to diminish the time of 

 vaporization of a given quantity of water at an assumed temperature below 

 the maximum. 



"8th. Though it has been shown that water thrown upon red hot metal 

 is adequate to produce explosive steam, even when it does not cool the me- 

 tal down to the temperature of most rapid vaporization, it is not the less true 

 that metal more than two hundred degrees below a red heat in the dark, is in 

 the condition to produce even a more rapid vaporization of water thrown 

 upon it than when red hot. 



These experiments show why steam is rapidly produced by red 

 hot metal, notwithstanding the great repulsion between the heated 

 metal and the water : for the metal requires to be cooled but two 

 hundred degrees below redness to reach the point of greatest vapor- 

 ization. 



The deductions as to the danger resulting from water obtaining 

 access to highly heated metal within a boiler, are supported by a re- 

 ference to two violent explosions, in one of which (on board of the 

 Grampus steamer) six cylindrical boilers exploded simultaneously. 

 The engineer had discovered, just before the explosion, that the boil- 



