30 Mr Sylvester on the [JuLy, 
same. That is, if 7 be the latent heat, and ¢ the tempera- 
ture,/ + ¢ = c aconstant quantity, whatever ¢ may be. I 
differ with Dr. Thomson as to any practical advantage derived 
from the variable quantity of latent heat at different temperatures 
either in distillation or in its agency in steam engines. Suppose 
in the former application that vapour is distilled over atthe tem- 
perature of 70°, and condensed in a temperature of 50°, a con- 
stant succession of liquid will be formed by condensation, which 
is the practical effect desired, and it must be admitted from the 
law quoted by Dr. Thomson, that the stock of vapour at 70° 
constantly passing from the still to the receiver will hold more 
latent heat than the same quantity at a higher temperature. The 
difference will consist in having a small excess of latent heat, 
which is “in the uncondensed vapour at every period of the 
process without any disadvantage,” as to the ultimate quan- 
tity of liquid condensed. I am inclined to think that if the 
size of the apparatus be increased so that the same weight of 
vapour may come over in the same time, the advantage would 
be in favour of the low temperature, owing to the quantity of 
heat lost in all processes carried on at high temperatures by 
radiation and the conducting power of contiguous bodies. 
For the same reasons there is no advantage in using steam 
for engines at a high pressure. Whatever may be the fuel con- 
sumed to make a given volume of steam equal to one atmosphere, 
it will take twice the quantity to give twice that volume, or the 
same volume of a density to give a pressure equal to two atmo- 
spheres. I should think therefore, that the increased tem- 
perature of the volume equal to two atmospheres would lose 
more heat to surrounding bodies than the two volumes of one 
atmosphere, but the mechanical advantage of the two will be 
obviously the same. The boasted advantage of the Cornish 
engines has chiefly arisen from their inventor assuming some 
erroneous data respecting the power of steam, and many others, 
even Mr. Herapath, seem to have fallen into the same mistake. 
In the range of temperature commonly used for high pressure 
steam, it will be found that from an increase of temperature of 
every 30° degrees, the density and elasticity of the steam 
become doubled ; that is, at 212°, its elasticity is equal to about 
30 inches of mercury, and a cubic foot of such steam would 
weigh about 253 grains. At 212 + 30 = 242 degrees, the 
volume remaining the same, it will support 60 inches of mercury, 
and a cubic foot will contain 253 x 2 = 506 grains. Hence it 
will appear that the temperature is increasing in arithmetical 
proportion while the power of the steam increases in geometrical 
Eepporion, and hence the apparent advantage by working with 
high pressure. * 
he source of this fallacy will be found in the assumption of . 
# The force of steam has not strictly a geometrical ratio to the temperature. The 
ratio for 10° below 212° is about 1°23. And this ratio for every ten degrees above 
-will decrease by -01, while steam, for every ten degrees below, bas a similar increase, 
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