Another method of ascertaining the quantity of sugar consists in adding to a given quantity of the fluid a known weight of yeast—having by experiment determined the quantity of fixed matter contained both in the yeast and in the fluid operated on. After fermentation is com plete the solution is evaporated to dryness, and the loss of solid matter sustained indicates the quantity of sugar which has passed off as 'alcohol and carbonic acid ; the urea likewise is de stroyed in the process, and its quantity must be deducted from the total loss. It is very desi rable when sugar is present in considerable quantity that the evaporation to dryness should be performed in vaeuo over sulphuric acid, as it is the only method that ensures our ob taining the sugar always in the same state of hydration. This process, however, is tedious, alvvays requiring many days for its completion : some compamtive experiments, showing the value of this precaution, will be detailed here after.
Our methods of exactly determining the quantity of sug-ar of milk are imperfect; I shall describe the plan which answers best when treating of the analysis of milk.
Uric acid.—This is one of the most inte resting and important animal products from the part it frequently performs in some of the most serious and distressing diseases to which we are liable. The method of detecting its presence by evaporation on glass with nitric acid has already been described. When no albuminous principles are present, the solution is evaporated to dryness and the residue treated with water, acidulated with hydrochloric acid ; the inso luble portion is dried and weighed, then burned ; the weight of the remaining ash (silica), if any, is deducted ; ,the loss indicates the quantity of uric acid.
Urobentoic, or, as it is often called, hippuric acid, has hitherto been found only in the urine, in which it generally exists in minute quantity, according to Liebig. Its quantity may be as certained by adding hydrochloric acid to the liquid concentrated by evaporation. It is eva porated to dryness by a water heat, and the residue digested with pure anhydrous ether as long a.s any thing is dissolved. By sponta neous evaporation it is left behind nearly pure, and may be weighed ; traces of urea crystallize with it, and a little of the odorous principle, which obstinately adheres to it. If the ether have dissolved any fatty matter, the addition of boiling water will dissolve the acid and leave the fats. Urobenzoic acid may be detected by heating a crystal in a test tube ; benzoic acid and benzoate of ammonia, and a few red drops of an oily matter, sublime, accompanied by a smell of bitter almonds and of the Tonka bean.
Alcohol dissolves it freely, and the solution by evaporation leaves stellated groups of needle shaped crystals. With perchloride of iron solutions of the urobenzoates furnish a cinna mon brown precipitate.
Lactic acul cannot be quantitatively deter mined without difficulty. The best method is as follows :—The solution if acid is neu tralized with ammonia, evaporated to dryness, and the residue exhausted with alcohol ; to the filtered liquid sulphuric acid is added drop by drop as long as a precipitate ensues; the bases are thus removed as sulphates. The solu tion is filtered and the precipitate washed wall alcohol; the clear liquid is digested with fre quent agitation, at a moderate heat, for twenty four hours, with carbonate of lead : hydrochloric, sulphuric, and phosphoric acids are thus sepa rated as insoluble salts, while lactate of lead dis solves. The solution is a second time filtered, sulphuretted hydrogen gas transmitted through the clear liquid, and the sulphuret of lead sepa rated by another filtration ; the filtered fluid is evapomted to dryness to expel the alcohol and ex cess of gas, the residue redissolved in water and digested with carbonate of zinc ; by evaporation of the filtered liquid crystals of lactate of zine are obtained. If the solution is now mixed with carbonate of potash in excess, evapormed to dryness, exhausted with boiling water and the residue dried and ignited, pure oxide of zinc is obtained: 100 grs. of this oxide ind icate 202.5grs. of lactic acid.
If oxalic acid be suspected in any liquid, it must be super-saturated with lime water. On treating the precipitate with acetic acid the phosphates dissolve, and oxalate of lime re mains, as the phosphate of iron is insoluble in acetic acid. The absence of iron must be determined by dissolving the precipitate in nitric acid, nearly neutralizing with ammonia and adding hydrosulphuret of ammonia ; the iron, if present, would fall as a black sulphuret, mixed with oxalate of lime. Oxalate of lime dissolves in dilute nitric acid, and, on super saturating with ammonia, is thrown down un changed. By convertin, the oxalate of lime into the carbonate or sulAate, as directed when speaking of the estimation of lime, the quan tity of oxalic acid may be inferred-100 grs. of the carbonate indicate of anhydrous oxalic acid 72 grs., and 100 of the sulphate 51.94 of the acid.