LATENT HEAT OF FLUIDITY. 285 



were employed, the rise in temperature would be still greater, the 

 capacity for heat of water being greater than that of almost all 

 other known substances. 



In measuring quantities of heat, what is taken as the " unit of 

 measurement" (as a foot, yard, or mile is taken as unit for 

 measures of length ; a pint, gallon, or cubic yard, &c., for measures 

 of bulk ; a second, hour, or year, &c., for measures of time ; and so 

 on) is the quantity of heat which suffices to raise the temperature of 

 a unit of weight of water (e.g., 1 Ib. of water) through the range of 

 1.* Hence the capacity for heat of a given substance as com- 

 pared with that of water may be represented as being the increase 

 in temperature that would be produced in a given weight of water 

 by the quantity of heat that would raise the temperature of the 

 same weight of substance 1. This value is spoken of as the 

 specific heat of the substance. For instance, the quantity of 

 heat that would raise the temperature of a given weight of copper 

 or zinc by exactly 1 C., would only raise the temperature of the 

 same weight of water about one tenth of a degree ; so that the 

 capacity for heat of copper or zinc is only about one tenth of the 

 capacity for heat of water ; and the specific heats of these metals 

 are consequently expressed by decimal fractions close to O'l. 



Expt. 330. Latent Heat of Fluidity. Weigh up a pound of 

 crushed ice and put it in a basin, and into another similar basin 

 put a pound (-f- of a pint) of water chilled down to C. by means of 

 pieces of ice, but riot containing any pieces of actual ice floating 

 about in it. Into each basin pour one pound of hot water, 

 measured or weighed out in a flask, and heated over a lamp until 

 it is just at the temperature 80 C. Mix up the water thoroughly 

 by stirring in each case, and then take the temperature with a 

 thermometer ; in the first case it will be found that the addition 

 of the hot water will suffice to melt all the ice after sufficient 

 stirring ; but the temperature of the resulting 2 Ibs. of water (one 

 added as such, the other produced by the melting of the ice) will 

 be very little above ; whilst in the second case the temperature 

 of the resulting 2 Ibs. of water will be about 40 C. Taking a 

 pound as the unit of weight, 1 Ib. of water at 80 will contain 80 

 units of heat more than the same water at ; none of this is 

 lost by simply mixing hot and cold water together, since 2 Ibs. of 

 water at 40 will still contain 2 x 40 = 80 units of heat more than 

 the same water at ; but when ice is melted by hot water, the 



* Strictly speaking, the amount of heat requisite to raise the temperature 

 of a given weight of water from to 1 is not quite the same as that 

 requisite to raise the temperature from 10 to 11 or from 50 to 51, and so 

 on ; but the difference is very slight. 



