EBULLITION. 



301 



The fact that the steam into which the water is converted contains a con- 

 siderable quantity of latent heat, and the computation of the exact amount of 

 that quantity will be more clearly understood if we compare the effects pro- 

 duced by mixing an ounce of water at 212 and an ounce of steam at 212, 

 respectively, with five and a half ounces of water at 32. We have seen that 

 an ounce of steam at 212, mixed with five and a half ounces of water at 32, 

 forms six and a half ounces of water at 212. Now, if one ounce of water at 

 212 be mixed with five and a half ounces of water at 32, the mixture will 

 have a temperature of about 60. In fact, the 180, by which the temperature 

 of the ounce of water at 212 exceeds the temperature of the five and a half 

 ounces of water at 32, are distributed through the mixture in the proportion 

 of the quantity of water, so that each of the five and a half ounces receives the 

 same increment of temperature ; and the loss of temperature which the ounce 

 of water at 212 sustains is equally divided among the other five and a half 

 ounces. Now, the mixture, in this case, having a temperature of only 60, 

 while, in the case where an ounce of steam at 212 was mixed with five and 

 a half ounces of water at 32, the mixture had the temperature of 212, it fol- 

 lows that the steam from which .the increased heat is all derived contains so 

 much more heat than the ounce of water at the same temperature as would be 

 necessary to raise six and a half ounces of water from the temperature of 60 

 to the temperature of 212, or six and a half times as much heat as would be 

 requisite to raise one ounce of water through about 152 of temperature. This 

 quantity of heat will, therefore, be found by multiplying 152 by six and a half, 

 which will give a product of 983, being nearly equal to the quantity of latent 

 heat determined by the former calculation. 



On a subject so important as the latent heat of steam, it may not be uninter- 

 esting here to mention some of the means by which Dr. Black, the discoverer 

 of latent heat, computed the quantity absorbed by water in its conversion into 

 vapor. 



If a given weight of water be exposed to a regular source of heat, and the 

 time required to raise it from the temperature of 50 to its boiling point be ob- 

 served, the rate at which it receives heat per minute may be computed. Let 

 the time be then observed which elapses from the commencement of the ebul- 

 lition to the total disappearance of the water ; and if it be assumed that in each 

 minute the same quantity of heat was communicated to the boiling water as 

 was comm.unicated before ebullition commenced, the quantity of heat carried 

 off by the steam may easily be calculated. Some water placed in a tin vessel 

 on a red-hot iron, was observed to rise from 50 to 212 in four minutes, being 

 at the rate of forty and a half degrees per minute. The same water boiled off 

 in twenty minutes. If it received during each of these twenty minutes forty 

 and a half degrees of heat, it must have carried off as much heat in the form 

 of steam as would be sufficient to raise water through twenty times forty and a 

 half degrees, or 810 ; a result corresponding nearly with the quantity of latent 

 heat already determined. 



If water submitted to pressure be raised to the temperature of 400, and the 

 mouth of the vessel which contains it be then suddenly opened, about a fifth 

 of the whole quantity of water will escape in the form of steam, and the tem- 

 perature of the remainder will immediately fall to 212. Thus the whole mass 

 of water has suddenly lost 188 of temperature, which is all carried away by 

 one fifth of the mass in the form of steam. Thus, the heat which has become 

 latent in the steam will be determined by multiplying 188 by five, which 

 gives a product of -940. The steam, therefore, is water combined with at 

 least 940 of heat, the presence of which is not indicated by the thermom- 

 eter. 



