The reaction with phenylhydrazin has not been much used for quantitive estimations of sugars, but it has been found especially useful in identifying and separating reducing sugars. It is altogether probable, however, that in the near future phenylhydrazin will become a common reagent for sugar work. Maquenne has studied the action of phenylhydrazin on sugars and considers that this reaction offers the only known means of precipitating these bodies from solutions where they are found mixed with other substances.[115] The osazones, which are thus obtained, are usually very slightly soluble in the ordinary reagents, for which reason it is easy to obtain them pure when there is at the disposition of the analyst a sufficient quantity of the material. But if the sugar to be studied is rare and if it contain, moreover, several distinct reducing bodies, the task is more delicate. It is easy then to confound several osazones which have almost identical points of fusion; for example, glucosazone with galactosazone. Finally, it becomes impossible by the employment of phenylhydrazin to distinguish glucose, dextrose or mannose from levulose alone or mixed with its isomers. Indeed, these three sugars give, with the acetate of phenylhydrazin the same phenylglucosazone which melts at about 205°. It is noticed that the weights of osazones which are precipitated when different sugars are heated for the same time with the same quantity of the phenylhydrazin, vary within extremely wide limits. It is constant for each kind of sugar if the conditions under which the precipitation is made are rigorously the same. There is then, in the weight of the osazones produced, a new characteristic of particular value. The following numbers have been obtained by heating for one hour at 100°, one gram of sugar with 100 cubic centimeters of water and five cubic centimeters of a solution containing forty grams of phenylhydrazin and forty grams of acetic acid per hundred. After cooling the liquid, the osazones are received upon a weighed filter, washed with 100 cubic centimeters of water, dried at 110° and weighed. The weights of osazones obtained are given in the following table: Weight of the osazones. Character of the sugar. gram. Sorbine, crystallized 0.82 Levulose ” 0.70 Xylose ” 0.40 Glucose, anhydrous 0.32 Arabinose, crystallized 0.27 Galactose ” 0.23 Rhamnose ” 0.15 Lactose ” 0.11 Maltose ” 0.11