148 THE ACTION OF HEAT. 
heat at the closing stage of the evaporation is capable of 
producing this disastrous result when the great mass of the 
syrup is of a much lower temperature than that at which 
caramelization of the sugar occurs. The cause of this is 
to be found in the imperfect convection of the heat through 
the sluggish mass of half liquid syrup. It is not conveyed 
away as fast as it is received by that portion of the syrup 
which is directly in contact with the bottom of the pan. 
Hence the necessity of placing the finishing pan at that 
extremity of the evaporating range which is farthest re- 
moved from the furnace. The pan, too, should be shallow, 
in order that the evaporation may be rapid, and that the 
transmission of the heat through it may be as rapid as its 
absorption by the lower stratum of the liquid. It should 
be provided with a thermometer capable of indicating at 
any moment to the eye the temperature, and thence the 
density, and the pan should be so adjusted as readily to 
admit of the instant removal of its contents from exposure 
to the heat, and, with equal facility, of being emptied, re- 
placed and refilled. Such a finishing pan should secure 
the best results attainable by a system of open-air evapo- 
ration, and the form which most fully meets those require- 
ments in practice, and seems best adapted to common use, 
is that of the tilt pan. (D. See engraving.) It is merely 
a modification of the old French bascule, a vessel long in 
use among sugar manufacturers in Mauritius, Guadaloupe, 
and elsewhere. 
A prolonged heat is exceedingly injurious to a solution 
of sugar. ‘The control of heat over the chemical affinities 
which determine the form and sensible properties of most 
substances, and especially of organic bodies which are 
isomeric, is well known. Sorghum juice contains a number 
of substances which are peculiarly unstable in their nature. 
