308 



EBULLITION. 



whatever pressure the water may be boiled. It follows also that, in the steam- 

 engine, equal weights of high-pressure and low-pressure steam are produced 

 by the same consumption of fuel ; and that, in general, the consumption of fuel 

 is proportional to the quantity of water vaporized, whatever the pressure of 

 the steam may be. 



The quantity of heat consumed thus depending on the weight of water evap- 

 orated, it is obviously a point of considerable practical importance to determine 

 the specific gravities or densities of steam raised under different pressures, 

 and at different temperatures ; yet this is a point on which even philosophical 

 authorities, in general entitled to respect, appear to have fallen into error. It 

 has been stated that the specific gravity or density of steam is always propor- 

 tional to its pressure.* This, however, is not correct. The true law for the 

 variation of the density or specific gravity of steam is the same as that of air : 

 it is proportional to the pressure or elasticity, provided the temperatures are the 

 same. If, then, we have steam raised from water under two different pres- 

 sures, and at two different temperatures, let the temperatures be equalized by 

 applying heat to the steam of the lesser pressure out of contact with water, its 

 pressure being meanwhile preserved. When the temperatures are thus ren- 

 dered equal, then their densities or specific gravities will be in the same pro- 

 portion as their pressures. 



If the space below the piston P, in the cylinder A B, fig. 4, be completely 



Fig. 4. 



filled with water, and a sufficient force be exerted on the piston to prevent it 

 from rising in the cylinder, the water under it may be heated to any required 

 temperature ; because, no space being allowed for the formation of steam, no 

 heat can become latent, and therefore all the heat communicated to the water 

 will be effective in raising its temperature. If the temperature of the water 

 under these circumstances were raised until it attained the limit of 1,212, it 

 would have all the heat necessary to give it the vaporous form, no part of that 

 heat being in this case latent. In fact, the water would, under such circum- 

 stances, be converted into vapor, in which the whole of the heat would be 

 sensible, and which would have no latent heat except such as the water pos- 

 sessed in the liquid state. If the piston, under these circumstances, be raised, 

 the water, or rather steam, below it, will expand ; and as it expands, its tem- 

 perature will fall, a portion of the sensible heat becoming latent. If the piston 

 were raised until the space below it were increased seventeen hundred times, 

 the steam would fall to the temperature of 212, and 1,000 of heat would be- 



* Thomson on Heat and Electricity, p. 221. 



