carbonic acid set free is not sufficient to totally prevent the precipitation of protoxyde of mercury, and a brownish-yellow precipitate is produced. This appears to be the reason why the presence of a certain amount of chloride of sodium defers the indication of the complete precipitation of urea, and why a further increase in the amount of chloride of sodium does not interfere with the test after having reached a certain height.

In operating upon urine containing from 1 to 15 per cent. of chloride of sodium, the number of milligrammes of urea contained in 100 c.c. of urine may at once be correctly obtained by simply subtracting 2 c.c. from the total cubic centimetres of mercurial solution used. The results thus obtained are relatively correct as regards the differences of the amounts of urea, even when the quantity of chloride of sodium varies in the urine of different individuals. There is only a slight error in the absolute quantity of urea, which, if left uncorrected, may amount to 15 or 20 milligrammes in 10 c.c. of urine.

If, therefore, we require to know the absolute quantity of urea in urine, the chlorine must be removed from the urine, and the chloride of sodium converted into the nitrate of soda. This is done by the agency of a graduated solution of nitrate of silver. 11 601 grammes of fused nitrate of silver are dissolved in water, and diluted until the volume of the fluid amounts to 400 c.c. One cubic centimetre of this solution contains 29.01 milligrammes of nitrate of silver, corresponding to 10 milligrammes of chloride of sodium.

The solution of mercury which will be described under the head of chloride of sodium corresponds to this solution of silver. Equal volumes of both will on use indicate equal quantities of kitchen salt. If, therefore, to 10 c.c. of urine we had to add 12.5 c.c. of the mercurial solution just alluded to, for producing the turbidity indicating that all the chloride of sodium is converted, then 12.5 c.c. of the solution of silver, on being added to 10 c.c. of urine, will precipitate the whole of the chlorine without any silver being left in solution.

As, by means of the mercurial solution, we can ascertain in a few seconds how much of the solution of silver it is necessary to add to urine containing chloride of sodium, for the purpose of removing the latter, this operation, which otherwise would be laborious and take much time, is divested of all inconvenience.

Let us suppose that, for 15 c.c. of urine precipitated with the solution of baryta, corresponding to 10 c.c. of the original

urine, we have used 17.5 of the mercurial solution graduated for kitchen salt. We now measure with a pipette

300 c.c. of the same urine, add

350 c.c. of the solution of silver,

65.0 c.c.

and throw the mixture on a filter.

Of the filtered liquid we now take for the test of urea always one half of the number of cubic centimetres of the mixed fluid, viz., 32·5 c.c., in which there are contained 10c.c. of urine, less phosphates and chlorides. These are now mixed with the mercurial solution graduated for urea; and the quantity of the latter is thus ascertained, regard being always had to the dilution in consequence of the addition of the solution of silver.

Modification required by the urine containing ammonia.— For common urine, one volume of solution of baryta to two volumes of urine is generally sufficient for precipitating the whole of the phosphoric and sulphuric acid present, and leaving a small amount of baryta in solution. If, however, the urine becomes alkaline from the presence of an alkaline carbonate, which most commonly is carbonate of ammonia, from the decomposition of urea, one volume of solution of baryta to two volumes of urine is in most cases insufficient to precipitate the whole amount of carbonic acid. It therefore will be necessary to add a larger amount of the solution of baryta.

If three volumes of the solution of baryta are mixed with four volumes of urine, 175 c.c. of the filtered fluid, corresponding to 10 c.c. of urine, will have to be taken for the analysis of urea by precipitation. Of a mixture of equal volumes of solution of baryta and urine, 20 c.c. must be taken for the test; and so on in the same proportion.

The influence of the decomposition of urea will in many cases not prevent the same results from being obtained in putrid urine as were arrived at in the same urine when fresh. Two or three days' standing will generally make no difference; but after that, the analysis with the mercurial fluid cannot any longer be depended upon.

If exact analyses of ammoniacal urine be required, we may either fix the carbonic acid, and transform the urea present into the same acid and ammonia by subjecting a quantity of urine to Bunsen's analysis; or we must determine the ammonia and urea each by a separate process in two separate portions of urine, and calculate the amount of urea from the ammonia by which it is represented.

For the analysis of urea in this kind of urine, it is not precipitated with the mixture of solution of baryta, but with baryta water only. From the filtered fluid a portion is taken corresponding to 10 c.c. of urine, and heated in a water bath, until ammonia is no longer evolved. This expulsion of ammonia is easily effected, because, after the addition of baryta water, which, when added in sufficient quantity, combines with the whole amount of carbonic acid present, the whole amount of ammonia is contained in the form of caustic ammonia. In the fluid thus freed of ammonia, urea is determined by the mercurial fluid.

In another portion of the urine the ammonia has to be determined by one of the ordinary alkalimetrical methods. The most convenient test is dilute sulphuric acid, which may be procured by mixing 16:333 grammes of pure hydrated sulphuric acid with as much water as will raise the whole of the mixture to 500 c.c., or 1000 half c.c. Of this sulphuric acid 0.5 c.c. exactly saturate 5.66 milligrammes of ammonia, being the quantity produced by the decomposition of 10 milligrammes of urea. Every cubic centimetre of sulphuric acid, therefore, used for neutralizing the ammonia of urine, corresponds to 20 milligrammes of urea originally contained in the urine.

The analysis becomes more accurate if we subject a known quantity of urine (after treatment with baryta) to distillation, collect the product in a receiver containing a known volume of the graduated sulphuric acid, which must be more than sufficient for neutralizing the whole amount of ammonia that passes over. The quantity of free acid left is then determined by means of a dilute solution of ammonia, graduated upon the dilute and graduated sulphuric acid. If, for example, we have put into the receiver 40 c.c. of dilute sulphuric acid, and if, after partial saturation by the distillate, we yet require 150 c.c. of the graduated solution of ammonia for neutralizing the acid, then a quantity of ammonia has passed over by distillation which has neutralized 40-15 = 25 c.c. of the graduated sulphuric acid, and represents 250 milligrammes of urea.

Modification required by the presence in urine of certain nitrogenized matters not being urea.-Besides urea some other bodies are precipitated by the solution of nitrate of protoxyde of mercury. One of them is allantoine, as has been shown by Dr. Limpricht. It has been stated by Professor Städeler that he found allantoine in the urine of

Annal. d. Chem. und Pharm.,' 1853, October, p. 99.

Allan

dogs labouring under difficulties of respiration. toine is present in the urine of the sucking calf; but it has never as yet been found either in the normal or pathological urine of man. Even under circumstances where its occurrence was most probable, as after the ingestion into the stomach of uric acid or urate of ammonia, when the usual products of the decomposition by oxydation of that body are found in the urine, viz., urea (an excess) and oxalic acid, the third produce of the artificial process, allantoine, could not be found in the urine even by such a chemist as Wöhler. We may therefore be quite safe that our analyses will not easily be made inaccurate by the presence of that body.

There are, however, some nitrogenized matters, forming an insoluble precipitate with the nitrate of mercury, which are of more frequent occurrence, particularly in the urine of patients. Kletzinsky,2 in his comparative experiments on the value of different methods for determining the quantity of urea, found that there is a substance present in urine, which, by its property of being precipitated by a solution of sugar of lead, manifests itself as different from urea, and yet is precipitated along with urea by the mercurial solution. This substance, in five experiments made upon healthy urine, amounted to 2, 2, 3, 3, and 4 per cent. of the urea; but in the urine of patients it would rise to 12 per cent. of the urea present. Professor Vogel is of opinion that the error in some cases might amount to 20 per cent. of the urea present.

This error must be avoided by the following proceeding:

Of a solution of sugar of lead, acidulated with a few drops of acetic acid, a sufficient quantity is added to urine to precipitate the whole of these nitrogenized matters. Some of them are nearly related to the colouring matter of urine, which is itself precipitated by acetate of lead. Any excess of the solution of lead must be removed by a current of sulphuretted hydrogen. After filtering, the urine may be used for the test for urea, due allowance being made for the dilution.

Bunsen's method of ascertaining the absolute quantity of urea in urine.—I have already stated the principle of this accurate method. Its advantages are, that it may be used

'Annal. d. Chem. und Pharm.,' Bd. 65, pp. 340-341.

2 Heller's Archiv,' 1853, p. 252.

3 Loc. cit., p. 244.

Annal. d. Chem. und Pharm.,' Bd. 65, p. 375.

for any description of urine, without requiring either correction or modification, as neither uric, hippuric, or benzoic acid, nor sugar, albumen, colouring or other nitrogenous matters, exert any influence over the results.

Thirty or forty grammes of urine are weighed or measured into a balloon; from 8 to 10 grammes of a very concentrated solution of chloride of barium mixed with a little ammonia are added; the balloon is closed with a cork, and the mixture is well shaken. After it has stood until the precipitate has settled to the bottom, the mixture is filtered through a filter of known weight on a funnel, the tube of which is drawn to a long, thin point. Of the clear fluid, 25 to 30 grammes are allowed to run into a strong glass tube, closed at one end, and containing about 3 grammes of solid, chemically pure chloride of barium. The weight of the tube and contents is determined before filling. The walls of this tube round the upper aperture must be kept quite dry. The weight of the urine filled into the tube is then ascertained, after which the tube is closed by the blowpipe, 1" or 1" above the level of the fluid. The tube so closed is now put into an oil bath, and for three or four hours exposed to a temperature of from 428° to 468° F. (220° to 242° C.) After the tube has again cooled, it is opened by means of a file and live charcoal; the crystals of carbonate of baryta are placed on a filter, washed carefully with water which is free of carbonic acid, and, after drying, are weighed. One part of carbonate of baryta corresponds to 04041 of urea.

The precipitate formed on mixing the urine with the ammoniacal solution of chloride of barium, and which remained on the filter, is then also washed, dried at 212°F. (100° C.), and weighed. The weight of this precipitate, subtracted from the weight of the urine and solution of chloride of barium, gives the total weight of the fluid, of which we have put from 25 to 30 grammes into the glass tube. From the urea found in the latter, that contained in the whole or any amount of urine may be calculated.

It is shorter to precipitate the phosphates and sulphates with the mixture of solutions of baryta water (two volumes) and nitrate of baryta (one volume), as described for Liebig's method. Of the filtered fluid a known quantity by measure, say 250 c.c., is poured into the tube. The precipitate on the filter is left out of the question, and thereby a great deal of trouble in washing and weighing is saved. The analysis certainly is thereby made less accurate, but so trifling is the error, that it may safely be allowed to pass.