The mode of formation of glucose, whether in the laboratory or in the organisms of plants and animals, requires some notice. It can be obtained chemically from starch and.from dextrine by boiling them with dilute sulphuric acid, or by the action of dias tase (q.v.), and from cellulose and gum, and from most of the varieties of sugar, by treatment with dilute acids. In the liver, it is formed: from the glycogen (q.v.) which occurs in that organ, under the influence of a ferment which has been chemically sepa rated from the hepatic tissue, but with the nature of which we are not acquaintea, while in the rest of the organism it is formed from the starch which is taken with the food, the starch undergoing this transformation under the influence of ptyaline (a fer nent occurring in the saliva); pancreatine (a ferment occurring in the pancreatic juice), and an unknown but corresponding ferment existing in the intestinal fluid. • The simplest method of preparing pure glucose is by treating honey with cold recti fied spirit, which extracts the uncrystallizable sugar; the residue is dissolved in water, and the solution is decolorized with animal charcoal, and aljowed to crystallize.
It is manufactured on a large scale, especially on the continent, from starth. A mixture of starch and water at a temperature of about 130° is made to flow gradually into a vat containing water acidulated with 1 per cent of sulphuric acid, and kept at the boiling-point. In about half an hour, the starch is converted into sugar. The liquid is drawn off, and the sulphuric acid is neutralized by the gradual addition of chalk, till there is no longer any effervescence. The sulphate of lime is deposited, and the clear aqueous solution, after being concentrated by evaporation, is set aside to crystallize. 'The molasses is drained off, and the sugar is dried at a gentle heat in a current of air. " The chief use," says Dr. Muspratt, in his Chemistry Applied to Arts and Manufactures, which glucose is applied On the continent, is for the manufacture of beer and a coarse kind of alcohol, which is said to be extensively converted into French brandy by the addition of oil of raisins, coloring matter, etc."
As all alcoholic drinks (ales, wines, and spirits) are obtained from fluids containing this variety of sugar as the essential constituent, and as their quality mainly depends upon the amount of sugar that is present, it is very important to have some ready means of determining its amount. A similar determination is also of great value in reference to the urinary secretion in diabetes, as it is mainly by ascertaining whether the daily amount of excreted glucose is diminishing or increasing that we can trace the favorable or unfavorable progress of the case.
Without entering into details, we may mention that there are three different modes of determining the amount of glucose in a fluid: the first is by determining the specific gravity; the second is the optical test, which is based upon the fact (already noticed), that solutions.of sugar (whether grape, cane, or milk sugar) exert right-handed rotation upon a ray of polarized light, the angle of rotation being proportional to the percentage of sugar. Soleil's apparatus for determining sugar in this way is described in the article POLARIZING APPARATUS. The third is by chemical means, of which the most important are Barreswil's method and the fermentation test. Barreswil's method is based upon the property which glucose possesses of throwing down suboxide of copper from alka line solutions of oxide of copper.
In employing the products of the fermentation of glucose as a means of determining its quantity, we take a given quantity of the saccharine fluid, add a little well-washed yeast, and collect the carbonic acid that is evolved over mercury. Roughly speaking, a cubic inch of carbonic acid corresponds to a grain of sugar.
Much information upon the different tests for glucose, and upon their relative degrees of delicacy, will be found in a paper recently published by Dr. Bence Jones in the Quar terly Journal of the Chemical Society, 1861, vol. xiv. p. 22.