=108. General Principles.=—The methods for the chemical estimation of sugars in common use depend on the reducing actions exerted on certain metallic salts, whereby the metal itself or some oxid thereof, is obtained. The reaction is either volumetric or the resulting oxid or metal may be weighed. The common method is, therefore, resolved into two distinct processes, and each of these is carried out in several ways. Not all sugars have the faculty of exerting a reducing action on highly oxidized metallic salts and the most common of them all, _viz._, sucrose is practically without action. This sugar, however, by simple hydrolysis, becomes reducing, but the two components into which it is resolved by hydrolytic action do not reduce metallic salts in the same proportion. Moreover, in all cases the reducing power of a sugar solution is largely dependent on its degree of concentration, and this factor must always be taken into consideration. Salts of copper and mercury are most usually selected to measure the reducing power of a sugar and in point of fact copper salts are almost universally used. Copper sulfate and carbonate are the salts usually employed, and of these the sulfate far more frequently, but after conversion into tartrate. Practically, therefore, the study of the reducing action of sugar as an analytical method will be confined almost exclusively to the determination of its action on copper tartrate. Direct gravimetric methods are also practiced to a limited extent in the determination of sugars as in the use of the formation of sucrates of the alkaline earths and of the combinations which certain sugars form with phenylhydrazin. Within a few years this last named reaction has assumed a marked degree of importance as an analytical method. The most practical treatment of this section, therefore, for the limited space which can be given it, will be the study of the reducing action of sugars, both from a volumetric and gravimetric point of view, followed by a description of the best approved methods of the direct precipitation of sugars by such reagents as barium hydroxid and phenylhydrazin. VOLUMETRIC METHODS. =109. Classification.=—Among the volumetric methods will be given those which are in common use or such as have been approved by the practice of analysts. Since the use of mercuric salts is now practiced to a limited extent, only a brief study of that process will be attempted. With the copper methods a somewhat extended description will be given of those depending on the use of copper sulfate, and a briefer account of the copper carbonate process.