290 HISTORY OF SCIENCE. 



place until the last particle of tin has been melted, when the tempera- 

 ture again begins to rise. The quantity of heat which thus becomes 

 latent in changing a body from the solid to the liquid form is called 

 its heat of fusion, and varies from one body to another : thus, while 

 the conversion of i lb. of ice into water requires 143 heat-units, i Ib. 

 of solid tin is changed into i lb. of liquid tin by the absorption of about 

 26 units of heat. 



When a pan of cold water is set on a fire, a thermometer plunged 

 in the liquid will continue to rise until it reaches 212, at which point 

 the water begins to boil, and the thermometer ceases to rise. As heat 

 is, nevertheless, constantly passing into the vessel, it must become 

 latent in this case also ; and Black, who first studied this subject, found 

 that this latent heat is employed in converting the water at 212 into 

 steam, which possesses no higher a temperature. The steam must 

 have this latent heat extracted from it before it resumes the liquid 



FIG. 137. FIG. 138. 



form. The principle of the methods by which the latent heat of 

 steam may be estimated is very simple. Let A, Fig. 137, represent 

 a vessel containing a known weight of water, which we shall suppose 

 to be ii Ibs. Into this water dips the pipe c, communicating with 

 the vessel B, which contains water that is made to boil by the appli- 

 cation of heat. The steam passing along the pipe c comes into con- 

 tact with the cold water in A, and is there condensed into water, which, 

 of course, mixes with the rest of the liquid. The temperature of the 

 liquid rises gradually until it reaches 212, after which the steam 

 entering from c ceases to be condensed, but rises up through the 

 liquid in bubbles and escapes, just as common air would do if forced 

 through the pipe. If, at the moment this first occurs, the process be 

 stopped, and the water in A be weighed, the weight will be found in- 

 creased : thus, if ii Ibs. were the original weight of water at 32, the 

 weight now would be 13 Ibs., or ii Ibs. of water at 32 mixed with 

 2 Ibs. of steam at 212, produce 13 Ibs. of water &\. 212; so that the 

 2 Ibs. of steam will change into water without diminution of tempera- 

 ture ; but in the changing of its condition from gas to liquid, it parts 

 with sufficient heat to raise 1 1 Ibs. of water from 32 to 212". Hence 



