METHODS OF EVAPORATION. 827 



effect, either to retain more or less of the sugar present as molasses, or, 

 as is the case with some, to cause the crystallization of more of the 

 sugar than would have been obtained without them. Owing to this 

 tendency of a substance when crystallizing to build up a crystal of 

 pure material, it is found, generally, that the crystals of sugar, even 

 when formed in a solution highly colored, are perfectlv pure, and color- 

 less as glass, and that the color of the ordinary yellow sugar is due al- 

 most entirely to the molasses which adheres to the surfaces of each mi- 

 nute crystal. 



In the defecated juice there always remains a certain amount of 

 impurities, which may be removed in the early stages of evaporation ; 

 and it will always be found that, as the evaporation progresses, a scum 

 will form upon the surface, which may be easily removed by skimming. 

 This scum will continue to rise, until the juice has been reduced to 

 nearly one-half its bulk, or to a density of about 22° Beaume, specific 

 gravity 1.092. 



A solution of sugar will, upon protracted boiling, suffer more or less 

 inversion of the sugar : but it will be seen, from the experiments given 

 upon page 297, that so long as there is an excess of lime in the solu- 

 tion, this inversion will not take place : also, that the effect of the 

 heat, when lime is present, is the destruction of the glucose, with the 

 formation of highly colored compounds, which will darken the syrup 

 produced. In the production, then, of syrup, it is most desirable, in or- 

 der to avoid a high color, that the removal of the water be effected 

 as speedily as possible. 



METHODS OF EVAPORATION. 



The means by which the removal of the excess of water is accom- 

 plished are, in pi'inciple, the same, viz., the evaporation by heat; but 

 the methods employed are numerous. They may be classed as, 



1. Evaporation in open pans over the fire. 



2. Evaporation in open pans by steam. 



3. Evaporation from heated surfaces. 



4. Evaporation from surfaces by hot air. 



5. Evaporation in vacuum. 



Evaporation in open Pajis over the Fire. 



This method is the most primitive and the most inexpensive. It is 

 in use to a greater or less extent in every sugar producing country. 



The following applications of this method will illustrate its exten- 

 sive use. 



The so-called Jamaica train consists, generally, of four large hemi- 

 spherical copper boilers, mounted in brick work in a row, the fire being 



