38 



the crystallizing density. If in a sirup the molecules of sugar are 

 brought sufficiently near to each other through concentration the re- 

 moval of the dissolving liquid these molecules attract each other so 

 strongly as to overcome the separating power of the solvent, and they 

 unite to form crystals. Sugar is much more soluble at high than at 

 low temperatures, the heat acting in this as in almost all cases as a re- 

 pulsive force among the molecules. It is therefore necessary to main- 

 tain a high vacuum in order to boil at a low pressure, in boiling to grain. 

 When the proper density is reached, the crystals sometimes fail to ap- 

 pear, and a fresh portion of cold sirup is allowed to enter the pan. 

 This must not be sufficient in amount to reduce the density of the con- 

 tents of the pan below that at which crystallization may take place. 

 This cold sirup causes a sudden though slight reduction of tempera- 

 ture, which may so reduce the repulsive forces as to allow the attraction 

 among the molecules to prevail, resulting in the inception of crystalliza- 

 tion. To discover this requires the keenest observation. When begin- 

 ning to form, the crystals are too minute to show either form or size, 

 even when viewed through a strong magnifying glass. There is to be 

 seen simply a very delicate cloud. The inexperienced observer would 

 entirely overlook this cloud, his attention probably being directed to 

 some curious globular and annular objects, which I have nowhere seen 

 explained. Very soon after the sample from the pan is placed upon 

 glass for observation the surface becomes cooled and somewhat hard- 

 ened. As the cooling proceeds below the surface contraction ensues, 

 and consequently a wrinkling of the surface, causing a shimmer of the 

 light in a very attractive manner. This, too, is likely to attract more 

 attention than the delicate, thin cloud of crystals, and may be even con- 

 founded with the reflection and refraction of light, by which alone the 

 minute crystals are determined. The practical operator learns to dis- 

 regard all other attractions, and to look for the cloud and its peculiari- 

 ties. When the contents of the pan have again reached the proper 

 density another portion of sirup is added. The sugar which this con- 

 tains is attracted to the crystals already formed, and goes to enlarge 

 these rather than to form new crystals, provided the firs.t are sufficiently 

 numerous to receive the sugar as rapidly as it can crystallize. 



The contents of the pan are repeatedly brought to the proper density, 

 and fresh sirup added, as above described, until the desired size of 

 grain is obtained, or until the pan is full. Good management should 

 bring about these two conditions at the same time. If a sufficient num- 

 ber of crystals has not been started at the beginning of the operation to 

 receive the sugar from the sirup added, a fresh crop of crystals will 

 be started at such time as the crystallization becomes too rapid to be 

 accommodated on the surfaces of the grain already formed. The older 

 and larger crystals grow more rapidly, by reason of their greater at- 

 tractive forc^e, than the newer and smaller ones on succeeding addi- 

 tions of sirup, so that the disparity in size will increase as the work 



