50 THE NATURE AND [SECT. n. 



water. J Count Rumford, Mr. Southern, and Dr. Ure also made experiments on 

 this principle. 2 



Also water may be heated in Papin's digester to 400 without boiling ; because 

 the steam is forcibly compressed, and prevented from making its escape. If when 

 heated to 400 the mouth of the vessel be suddenly opened, part of the water 

 rushes out in the form of steam, but the greater part still remains in the form of 

 water, and its temperature instantly sinks to 212; consequently, 188 of heat 

 have suddenly disappeared. This heat must have been carried off by the steam. 

 Now as about one-fifth of the water is converted into steam, that steam must con- 



1 Watt's Notes on Robison's Mech. Phil. vol. ii. p. 7. 



2 This mode of conducting the experiment has sometimes led to erroneous results, through a 

 want of attention to the mode of calculation. The quantity of heat being measured by the specific 

 heat of water, let 



W be the weight of water used to condense the steam ; 

 t, its temperature after the steam has been condensed in it ; 

 t', the quantity its temperature is raised ; 

 w, the weight of steam ; 

 s, its specific heat when condensed ; 



and x the whole heat required for its formation into steam. 



Then the heat communicated to the water by the steam, is as its weight multiplied by the rise 

 of temperature, or W /. 



The condensed steam has the temperature t after the operation ; and as its whole heat was x w 

 before condensation, it must be t s w after. Therefore 

 W t' = x to t s w, 



W if 



or + t s = x = the measure of the heat that will form the weight 



10 



of steam w. 



If T be the temperature of the steam before condensation, and s' its specific heat ; then 



W t' 



1- t s T s' = the heat of conversion into steam ; 



w 



and it appears from experiment to be nearly a constant quantity for the same liquid. 



But it is usual to suppose the specific heat of equal weights of the steam and the liquid which 

 forms it to be the same, and then 



W t' 



s (T t) = the heat of conversion. 



w 



And for water where s=l, it becomes 



W t' 



+ t T = the heat of conversion. 



w 



Hence we have this RULE : Multiply the weight of the water used to condense the steam by the 

 increase of its temperature, and divide the product by the weight of the condensed steam. The 

 quotient will express the quantity of heat evolved from the steam, and by adding to it the tempe- 

 rature after condensation, the sum will express the total amount of heat ; from which, deducting the 

 temperature of the steam before condensation, there will remain the quantity of heat due to the 

 conversion into steam. 



