curial solution used in this way, the amount of urea present may then be calculated; a consumption of half the volume shows half the amount of urea, of twice the volume double the quantity of urea, to be contained in the fluid. The results obtained by this analysis are equal in accuracy to those obtained by the methods of Bunsen and Ragsky; but these latter methods, though giving excellent results, are not available for practical purposes, on account of the time and attention required to carry out the operations. If it be required to make many analyses of urea at one and the same time, the method of Bunsen may not take more time than that of Liebig above described, though the operator must be expert at weighing, and not lose much time with the scales, or this one proceeding will take as much time as the whole of the titre analysis. As the analysis of Bunsen is not only absolutely correct, but also a beautiful conception, I have lower down given it in full; that of Ragsky being, though good, not easy of application, is omitted. Preparation of the Solution of Mercury for precipitating Urea from Urine. Four grammes of pure urca are first dissolved in water, and this solution is diluted with water to exactly the bulk of 200 c.c. By dissolving four grammes of urea in 200 c.c. of water, 201.75 c.c. of solution would be obtained, being 1.75 c.c. in excess. Of the solution of nitrate of mercury, which is to serve for the purpose of precipitating urea from the urine, 20 c.c. are to be just sufficient to indicate exactly the amount of urea contained in 10 c.c. of the solution just described, namely, 200 milligrammes of urea; one cubic centimetre, therefore, of the mercurial solution must correspond with 10 milligrammes of urea. To this end, the solution of mercury must contain an amount of oxyde sufficient to produce, with 10 milligrammes of urea, the nitrate containing four equivalents of the oxyde of mercury, and further, it must contain a trifling excess of the oxyde of mercury, in order to indicate the complete precipitation of the urea. This is the case when, after the addition of the last drop of the 10 c.c. of the mercurial solution to the solution of urea, a solution of carbonate of soda produces a distinctly yellow-coloured precipitate. According to calculation, 100 milligrammes of urea require for precipitation 720 milligrammes of the oxyde of mercury (in the form of nitrate); but in order to produce a distinct reaction of oxyde of mercury in dilute solutions of urea, the 10 c.c. of mercurial solution necessary to precipitate the 100 milligrammes of urea must contain an excess of oxyde amounting to 52 milligrammes, or in all 772 milligrammes of oxyde. Every cubic centimetre of the solution, therefore, must contain an excess of 5.2 milligrammes of oxyde. The simplest mode of obtaining the test-fluid is by dissolving in a beaker-glass one part of pure metallic quicksilver in five parts of nitric acid of 1·425 specific gravity, and frequently adding a little nitric acid, keeping the mixture at a gentle heat, until the evolution of vapours of nitrous acid has entirely ceased. The solution is then evaporated on the water bath until it assumes the consistence of a syrup. This syrup is then diluted with water until 100 c.c. of this dilute fluid contain exactly 7.140 grammes of mercury. This is the case, if 100 grammes of mercury, after transformation into the nitrate of the protoxyde, are dissolved in so much water that the bulk of the solution amounts to exactly 1400 C.C. If we use for the preparation of the protoxyde the crystallized nitrate of the suboxyde of mercury, which may with greater facility be obtained pure and is more free from other metals than metallic mercury, the concentrated solution of the protoxyde obtained is of unknown strength. The quantity of protoxyde contained in it must therefore be determined; and this being done, the solution is to be diluted to the strength already stated. There are several methods of finding the amount of protoxyde of mercury contained in a solution of the nitrate. It may be found in a direct way by diluting a known volume of the concentrated solution or syrup with ten volumes of water, and precipitating the protoxyde of 10 c.c. of this solution by the addition of potassa. Or a precipitate of the sulphide of mercury may be obtained by mixing the nitrate with a solution of sulphate of soda, and decomposing the precipitated sulphate of protoxyde of mercury by a current of sulphuretted hydrogen. A third proceeding, which dispenses with scales, is the following: Episode: Mode of ascertaining the amount of protoxyde of mercury contained in a solution of nitrate of mercury.On mixing a solution of the nitrate of protoxyde of mercury with a solution of phosphate of soda, a white flocculent precipitate of phosphate of protoxyde of mercury is immediately produced, which, on being allowed to stand in the liquid, rapidly becomes crystalline. A solution of corrosive sublimate may, however, be mixed with the alkaline phosphate, without any turbidity being produced. If to the mixture of the two first-mentioned salts we add a solution of chloride of sodium, before the precipitate has had time to become crystalline, the latter immediately decomposes with the chloride of sodium, corrosive sublimate and phosphate of soda being produced; the precipitate disappears, and the fluid becomes perfectly clear. This test is the basis of the following method, by which the amount of protoxyde of mercury contained in a solution of the nitrate may be ascertained with tolerable accuracy. One equivalent of phosphate of mercury requires for its redissolution one equivalent of chloride of sodium. It follows from this that if we know the amount of chloride of sodium which it has been necessary to add for redissolving the phosphate of mercury, we also know the amount of protoxyde contained in the solution of the nitrate. As the equivalent of the chloride of sodium is only about one half of that of the protoxyde of mercury, a slight error in the addition of the chloride of sodium will be doubled in the calculation of the mercury. This analysis is therefore not so accurate as its reverse, the determination of chloride of sodium by the mercurial solution. But it is sufficiently accurate for the purpose here intended. Episode in episode: Preparation of the standard solution of chloride of sodium to be employed in ascertaining the amount of mercury in solution.-A saturated solution of chloride of sodium is first prepared by pouring water over pure, transparent rock salt in coarse pieces, and letting it stand for solution at a temperature of from 54° to 75° F. (12.2 to 23.9°C.) If the mixture be frequently shaken it will, after the elapse of twenty-four hours, be perfectly saturated, and in every case will contain an invariable amount of salt, viz., 3.184 grammes in every 10 c.c. The solution, after decanting and filtering, is ready for use. Of this solution we take with a pipette Whereby we obtain of dilute solution of chloride of sodium 20.0 c.c. 566.8 c.c. 586.8 c.c. Which in all contain 6368 milligrammes of chloride of sodium, viz., the amount contained in 20 c.c. of the saturated solution. In 10 c.c. of this dilute solution there are consequently contained 108.52 milligrammes of chloride of sodium, corresponding to 200 milligrammes of protoxyde of mercury (1 c.c. dilute solution=20 milligrammes of protoxyde of mercury). This calculation is based upon the equivalents of protoxyde of mercury=108, and chloride of sodium=58.6, which stand to each other in the same proportion as 200 of the oxyde to 108.52 of chloride of sodium. Mode of ascertaining amount of protoxyde, &c. (continued). -In order to determine with some degree of accuracy the amount of oxyde contained in a solution of the nitrate, the latter must not be too concentrated, partly because a larger bulk admits of more accurate measurement, partly because the end of the reaction is much more perceptible in dilute than in concentrated fluids. It is therefore desirable that the mercurial solution, which is to serve for the test, should in 10 c.c. not contain more than from 180 to 200 milligrammes of protoxyde of mercury. The following preliminary experiment is therefore made, for the purpose of ascertaining the concentration: 100 c.c. of the solution of chloride of sodium are mixed with 4.0 c.c. of a solution of phosphate of soda (the officinal salt) saturated at the ordinary temperature. To this mixture the mercurial solution is now poured from a burette, until a precipitate is formed which does not disappear on shaking the fluid. Let us suppose that we have used for that purpose 24 c.c. of the mercurial solution; they accordingly contain 200 milligrammes of oxyde; 10 c.c. of the solution therefore contain more than 800 milligrammes, when they should only contain 200 milligrammes at the outside. This solution, therefore, before the actual testing begins, must be diluted with three times its own volume of water. Of this dilute solution of mercury we now measure 100 c.c. into a beaker, add 40 c.c. of the above-mentioned solution of phosphate of soda, and pour from a burette the graduated or standard solution of chloride of sodium into this mixture, which is kept in constant agitation, until at last the white precipitate, which is formed on addition of the phosphate to the mercurial solution, is entirely redissolved. The addition to the mercurial fluid of the solution of phosphate of soda must be followed immediately by that of the chloride of sodium; for if we suspend the addition of the latter only for a few minutes, the phosphate of mercury becomes crystalline, and now either dissolves not at all, or with difficulty only. The solution of mercury, moreover, must not contain too much free acid. It contains the proper amount, if, after the addition of the phosphate of soda, the mixture no longer reddens litmus. If it has, however, an acid reaction, the mercurial solution must, previously to the testing, have a part of its acidity neutralized by the addition of a few drops of a solution of carbonate of soda, until basic salt begins to be precipitated, which may be redissolved by a drop or two of dilute nitric acid. As a matter of course the errors in this analysis are mainly due to our adding a drop or two more of the chloride of sodium than is actually required to dissolve the precipitates. The greater the quantity of solution of chloride of sodium added to a given bulk of the mercurial solution, the more oxyde this solution will appear to contain. The error, consequently, which we have just pointed out, increases the apparent real amount of oxyde contained in the solution. As the phosphate of mercury is slightly soluble in the fluid, and as, after all, the solution of chloride of sodium is graduated with regard to this error, the latter is generally very slight. If the proceeding be reversed by pouring the mercurial solution into a mixture of the solution of chloride of sodium and the alkaline phosphate, a slight excess of the mercurial solution must always be added, for the purpose of producing the precipitate, which is not permanent until the fluid has been saturated with the mercury. This proceeding, therefore, would give too low an indication of the amount of oxyde. These analyses become still more correct, if we combine both methods, and proceed in the following manner: (Method I.) We measure 10 c.c. of the solution of mercury into a beaker, add from 3 to 4 c.c. of the solution of phosphate of soda, and, taking care not to let the precipitate become crystalline, we immediately pour from the burette into the fluid the solution of chloride of sodium, until the precipitate has disappeared. Let us suppose that we have used for that purpose 12.5 c.c. of the solution of chloride of sodium, we now— (Method II.) Measure 125 c.c. of the same solution of chloride of sodium into a beaker, add from 3 to 4 c.c. of the phosphate of soda, and pour into this mixture from a burette the amount of the same solution of mercury, which is just necessary for the production of a commencing precipitate. Let us suppose that we have used of it 10:25 c.c., then its real strength is-There have been used for I. 100 c.c. of the merc. sol. 12.5 c.c. sol. of chl. of sod. 11. 10.25 c.c. ditto. 12.5 c.c. ditto. 20.25 c.c. merc. sol. 25.0 c.c. sol. of chl. of sod. As every c.c. of the graduated solution of chloride of sodium |