_Maltose_ and _levulose_ are isomers of dextrose. The former is prepared by the action of malt or diastase upon starch. It has a dextro-rotary power of 150° and its property of reducing copper salts is only about 60 per cent. of that of dextrose. It is converted into the latter compound upon boiling with dilute sulphuric acid. Levulose, as previously stated, is formed, together with dextrose, from cane sugar by treatment with dilute acids or with ferments. It turns the plane of a ray of polarised light to the left, its rotary power varying considerably at different temperatures. _Lactose_, or milk sugar, has already been referred to under the head of Milk. It is isomeric with cane sugar, possesses a dextro-rotary power (58°·2), and undergoes fermentation when mixed with yeast, and reduces alkaline copper solutions, but in a different degree from glucose. Many of the substances frequently enumerated as being used to adulterate sugar are at present very seldom employed. The usual list includes “glucose” (often meaning invert sugar), sand, flour, chalk, terra alba, etc. Loaf sugar is almost invariably pure, although its colour is sometimes improved by the addition of small proportions of various blue pigments, such as ultramarine, indigo, and Prussian blue. The presence of ultramarine was detected in about 73 per cent. of the samples of granulated sugar tested in 1881 by the New York State Board of Health. Tin salts[55] are also occasionally employed in the bleaching of sugar and syrups. Granulated sugar is asserted to be sometimes mixed with grape sugar, and powdered sugar has been found adulterated with flour and terra alba; but the varieties which are most exposed to admixture are the low grades of yellow and brown sugar, in which, however, several per cent. of invert sugar are normally present. Sand, gravel, and mites form a rather common contamination of raw sugar. From the year 1876 to 1881, 310 samples of commercial sugar were examined by the public health authorities of Canada, of which number 24 were reported as containing glucose, and 11 as of doubtful purity. Of 38 samples of brown sugar recently analysed by Dr. Charles Smart, of the National Board of Health, 9 were adulterated with glucose. From the investigations of A. L. Colby, Analyst to the New York State Board of Health, it was found that of the 116 samples examined, the white sugars were practically pure; whereas, of 67 samples of brown sugar, 4 contained glucose. Of 16 specimens of brown sugar, tested by a commission appointed by the National Academy of Sciences in 1883, 4 contained about 30 per cent. of this body.[56] Many varieties of sugar-house syrups, and the various forms of confectionery, are very extensively adulterated with artificial glucose. The average sugar-house syrup has the following composition:--