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XXXI. On the Means of producing a double Distillation by the 
same heat. By Smithson Tennant, Esq, F. R. S. 
Read June 30, 1814. 
It was first made known by the experiments of Dr. Black, 
and has since been confirmed by those of Mr. Watt and 
other philosophers, that the quantity of heat required for rais- 
ing the temperature of water from fifty degrees to that of the 
boiling point, is only about a sixth of that which is afterwards 
required for converting the boiling water into steam. As the 
steam itself is not hotter than the boiling water, the heat which 
it had absorbed was said by Dr. Black to be latent ; being 
merely employed in supporting the aerial state which the 
water had assumed. Whenever this steam is condensed, the 
heat which was latent again reappears, and in such consider- 
able quantity, that it has been found convenient for various 
purposes to employ the condensation of steam for heating 
other bodies. 
But though water may by this means be brought to the 
boiling point, it can not be raised above it, and therefore can- 
not be converted into vapour, so as to pass over by distillation* 
As soon as the steam has imparted to the water its own tem- 
perature, there is no longer any transfer of heat, and the 
steam passes through the water uncondensed. 
If, on the contrary, the steam did continue to condense, then 
the water would itself be converted into steam, and might by 
