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Page:Popular Science Monthly Volume 19.djvu/268

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THE POPULAR SCIENCE MONTHLY.

rated, at as low a temperature as possible, to the required concentration. If grape-sugar is to be made, the process of conversion is not stopped as soon as the starch has disappeared, but is carried on still further to a point which can only be determined by trial. After concentration it is conveyed into tanks, where the process of solidification begins and continues for several days.

Glucose, on the other hand, will not harden, whatever the degree of concentration may be, or, at least, if it do so, only partially and after many months.

The habit of bleaching both glucose and grape-sugar by means of sulphurous acid is sometimes practiced, but is reprehensible. By the oxidation of the sulphurous acid, free sulphuric acid is likely to occur in the finished product.

Glucose and grape-sugar are mixtures of several chemical substances. Starch, which is composed of six atoms of carbon, ten of hydrogen, and five of oxygen, when subjected to the action of dilute sulphuric acid, appears to undergo a molecular condensation and hydration. Among the substances formed may be reckoned dextrine, glucose, and a substance isomeric with cane-sugar. This latter substance appears to be one of the early products of conversion, and this is the reason that the poorly converted glucoses are sweeter than the well converted. It is only after prolonged boiling with dilute acid that the product becomes chemically homogeneous, with a constitution which is probably represented by the symbol C6H12O6H2O.

Glucose presents several anomalies when examined with polarized light. Its highest rotatory power is found when it is made with the least possible amount of conversion—i. e., when the process of conversion is stopped as soon as the starch has disappeared. Continued boiling with dilute acid causes a gradual decrease of rotatory power. It is only after six to eight hours* heating to a temperature of 104° C. that a constant rotatory power is reached. This power is only about half that exhibited by the glucose as a maximum. This minimum rotatory power, however, is greater than that possessed by cane-sugar.

Glucose, like many other bodies, has the property of reducing a hot alkaline copper solution and separating the metal as a red sub-oxide. This power in glucose is always inversely as the rotating power. I have shown this fully and conclusively in the paper already referred to. The relation between reducing power and rotating power is a constant one, and hence the percentage of reducing power can be calculated from the polarimetric observations. This, however, is of more interest to the practical chemist than to the general reader, and I therefore pass it by.

The question of most practical importance is, "Is glucose a wholesome article of food?" I do not hesitate to answer this question in the affirmative, I mean by this, however, a glucose which is properly made. Such a glucose contains only a very little sulphuric