Dr. Davey's method of determining the amount of urea in urine.'-This method is founded on the fact that urea is very readily decomposed by the hypochlorites of soda, potash, or lime, and its constituent nitrogen is evolved in the gaseous state. From the quantity of gas evolved the amount of urea is estimated.

A strong glass tube, about twelve or fourteen inches long, closed at one end, and its open extremity ground smooth, and having the bore not larger than the thumb conveniently can cover, holding from two to three cubic inches, each divided into tenths and hundredths by graduation on the glass, is filled more than a third full of mercury, to which afterwards a measured quantity of urine to be examined is poured, which may be from a quarter of a drachm to a drachm or upwards, according to the capacity of the tube; then holding the tube in one hand near its open extremity, and having the thumb in readiness to cover the aperture, the operator fills it completely full with a solution of the hypochlorite of soda (taking care not to overflow the tube), and then instantly covers the opening tightly with the thumb, and having rapidly inverted the tube once or twice, to mix the urine with the hypochlorite, he finally opens the tube under a saturated solution of common salt in water, contained in a steady cup or small mortar. The mercury then flows out, and the solution of salt takes its place, and the mixture of urine and hypochlorite being lighter than the solution of salt, will remain in the upper part of the tube, and will therefore be prevented from descending and mixing with the fluid in the cup. A rapid disengagement of minute globules of gas soon takes place in the mixture in the upper part of the tube, and the gas is there retained and collected. The tube is then left in the upright position till there is no further appearance of minute globules of gas being formed, the time being dependent on the strength of the hypochlorite and the quantity of urea present; but the decomposition is generally completed in from three to four hours; it may, however, be left much longer, even for a day if convenient, and having set the experiment going, it requires no further attention; and when the decomposition is completed, it is only necessary to read the quantity of gas produced off the scale on the tube. In cases where great accuracy is required, due attention must be paid to the temperature and atmospheric pressure, and certain corrections made if these should deviate from the usual standards of comparison, at the time of reading off the volume of the gas; but in 1 'Philosophical Magazine,' June, 1854, p. 385; also Dublin Hospital Gazette,' June 1, 1854.

most cases sufficiently near approximations to accuracy may be obtained without reference to those particulars.

From a number of experiments, Dr. Davey has ascertained that the quantity of gas evolved from different amounts of urea, treated in the way just described, very closely approximates to the quantity of nitrogen gas which should be furnished from the urea by calculation. Thus, the fifth part of a grain of urea should furnish by calculation 0·3098 parts of a cubic inch of nitrogen gas at 60° F.o (15·5° C) and 30" bar. ; the same quantity of urea treated as described furnished in one experiment 0.3001, and in another 0.3069 parts of a cubic inch of gas at the same temperature and pressure; which shows that the calculated quantity of nitrogen differs from the amount of nitrogen gas obtained by only a few thousandths of a cubic inch.

If the relation that exists between a certain quantity of urea and nitrogen be known, we can, from the quantity of gas evolved in any case, easily calculate the amount of urca present by the simple process of rule of three. Thus the fifth part of a grain, or 02 of a grain of urea, gives by calculation. 0-3098 parts of a cubic inch of nitrogen gas. Then 0.3098: the volume of gas found: : 02: the required quantity of urea; or, multiplying the first and third terms by 5, we have 1·549 cubic inches of gas representing one grain of urea, which is a simpler proportion.

Comparative experiments, which Dr. Davey made with his method and that of Liebig, seemed to agree very closely. In each of the following experiments one fluid ounce urine yielded the respective quantities of urea in grains.

In the first and second experiments with Liebig's test, the quantity of common salt present in the urine was taken into account, as it increases the apparent quantity of urea in the urine. In the third, this was not taken into consideration, and the quantity of urea was compared with the mean of two results obtained by Dr. Davey's method.

The hypochlorite of soda is preferable to that of potash as a decomposing agent, because the soda salt is an article of the pharmacopoeia under the name "soda chlorinatæ liquor," and therefore can be easily procured; whereas the potash salt, not being used in medicine, would require to be specially made for the purpose. The hypochlorite of lime is not so

effectual, and has the disadvantage of soiling the sides of the graduated tube by the carbonate of lime formed in the

reaction.

In reference to the quantity of hypochlorite of soda to be employed, it should always be used in excess. About five or six times the volume of the urine employed will be found generally to be quite sufficient, and ensure there being an excess of the hypochlorite. The amount required may be easily determined also by direct experiment, by adding to a certain quantity of urine to be examined in a glass, a measured quantity of the hypochlorite, and leaving it for a short time till the evolution of gas is nearly over; then if, on the addition of more of the hypochlorite, the effervescence is renewed, it shows that there was not enough of the decomposing liquor first employed, and more must be added from time to time till no further evolution of gas is produced, and the quantity of hypochlorite used to arrive at this point indicates the amount necessary. One grain of urea requires somewhere about half a fluid ounce of the ordinary soda chlorinatæ liquor for its complete decomposition. The amount of mercury employed should, as a general rule, be never less than the volume of gas produced; for if the volume of gas evolved be more than that of the mercury used, it will be more than that of the solution of salt, and therefore some of the mixture of urine and hypochlorite will be forced out of the tube before it is completely decomposed, and consequently some of the gas will be lost; so that, if this occur, we must repeat the experiment, using either a larger quantity of mercury, if our tube will allow, or diminishing the quantity of urine employed.

It might be supposed, on first sight, that this method would be liable to the following source of error, viz., that some of the gas would be evolved and lost during the pouring in of the hypochlorite; but this is not the case, as several seconds elapse before there is any apparent reaction or evolution of gas on mixing the hypochlorite with the urine, and there is therefore full time to perform the experiment without any loss of the gas. Dr. Davey has also ascertained that the acid reaction of the urine does not affect his method. But there are some slight sources of error connected with it, the principal one being, that ammonia, if it exist in the urine, gives rise to nitrogen gas, and therefore increases the apparent amount of urea: but the same objection holds equally in Liebig's and Ragsky's methods, the two most accurate at present known. Uric acid also is similarly affected by the hypochlorite; but it and ammonia ordinarily occur in such

small proportion in urine, that the error produced from these substances would be but trifling, and is partly corrected by taking the calculated quantity of nitrogen, which is, as has been shown, something more than that obtained from a certain quantity of urea by direct experiment.

In cases where ammonia or uric acid occur in more than ordinary quantity, these substances must be separated by the usual means employed before having recourse to this method. This object would be effected by gently heating the urine with a certain quantity of baryta water as long as the odour of ammonia is disengaged, and then filtering the solution (see p. 66).

Another source of error is that, if a solution of the hypochlorite of soda alone, or standing over mercury, be exposed to the light for several days, it will very gradually evolve a minute quantity of oxygen, which shows that in determining urea we should not allow the experiment to go on for too long a time; but, if left for a day, or even two, it will scarcely make any appreciable effect on the quantity of gas evolved in testing for urea.

The reaction which appears to take place in the process seems to be the following: The hypochlorite of soda acting on the urea gives rise to the formation of carbonic acid, water, and chloride of sodium, together with the evolution of nitrogen gas. Thus

C2HÂN2O2 + 3(Cl,O. NaO) = 2CO2+4HO+3C1 Na +2 N.

The nitrogen is evolved, and the carbonic acid is absorbed by some of the hypochlorite of soda in excess, which absorbs carbonic acid very quickly without evolving any other gas; and Dr. Davey failed in several experiments to detect the smallest portion of carbonic acid in the gas produced by acting on urea, though he always noticed the presence of a very minute quantity of oxygen in the nitrogen gas.

Dr. Davey has also ascertained that several of the substances found in urine during discase, as, for example, sugar, albumen, biliary and excess of urinary colouring matter, produce scarcely any effect on the results obtained by this method.

We here give the literature of other methods for ascertaining the quantity of urca in urine:

Lecanu, Journal de Pharm.,' vol. xvii, p. 651.—In the form of nitrate.

Heintz, 'Poggend. Ann.,' vol. lxvi, p. 114.; vol. lxviii, p. 393; Ragsky, Annal. d. Chem. und Pharm.,' vol. liv, p. 29.In the form of (sulphate of) ammonia.

Millon, Compt. rend.,' vol. xxvi, p. 119.—In the form of carbonic acid and nitrogen.

Draper, Journ. of Pract. Chem.,' vol. lx, p. 381.

Physiological quantity of Urea.

Numerous experiments have shown that a healthy man, who lives well, discharges on an average from 30 to 40 grammes of urea in twenty-four hours, which, calculated upon one hour, gives 1.25 to 1.66 grammes. This average must of course vary a little according to the size of the individual 1; and in the individual it must be dependent on accidental circumstances, which will sometimes change it, and in rare instances will produce extreme maxima and minima. But for practical purposes the above figures are valuable, even though subject to the variations mentioned. It would be a much better basis for comparative researches at the bedside, if the average amount of urea could be expressed in proportion to certain units of weight of the body, say pounds or kilogrammes, or units of measure of length of body, say centimetres. But many experiments will be requisite before such a basis can with safety be given.

It would be useless here to enter into any speculation or theory on the mode and place of formation of urea in the system. We leave it undecided whether it is made in the blood, or blood-corpuscles, or in the muscles, however important the decision of these questions may be. But one great fact is undoubtedly established; namely, that as urea is the principal product of the metamorphosis in the body of nitrogenized food, the quantity of urea must stand in a direct relation to the quantity of food taken; or, if little or no food be taken, to the amount of nitrogenized component parts of the body disintegrated in the place of food. In this sense must be taken the expression that urea is the measure of dissimilation, if I may be allowed to use this term as the antithesis of assimilation. "Der Harnstoff ist das Maass des Stoffwechsels," says Bischoff. And this is so nearly true that (with a slight modification of the sentence) we may say-the amount of urea is the measure of the most important part of the change of matter in the system. The intensity of the change is expressed by the amount of urea in the urine. More urea is produced during waking, than during sleep; more during