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subject in the living body. To a patient who passes urate of ammonia, we may give acid medicines and cause a deposit of uric acid in its place, whilst litmus paper will be far more strongly reddened than it previously was.

For the above reasons, therefore, it seems most probable that when uric acid alone is deposited, much free acid must have been thrown out by the kidneys, and that thus all the urate of ammonia, which would otherwise have been present, must have been decomposed. If we wish to know how often in calculous complaints this highly acid state of secretion occurred, we must not only observe how often whole calculi consist of uric acid, but how often whole layers of this substance occur. This appears from the Table to have been 97 times in 450.

It was above stated that when little acid was added to urine or taken by a patient subject to a deposit of urate of ammonia, that substance was only partly decomposed ; and the conclusion which must be drawn from this seems to be, that when urate of ammonia is found mixed with uric acid, but little free acid is secreted by the kidneys. Such a mixture was found to occur in 38 layers. Hence in 38 states out of 450, but little free acid was thrown off in the urine.

When we find urate of ammonia alone, without any uric acid, forming a calculus or layer, we must consider that no free acid was removed by the kidneys, although the secretion may have been acid to test paper.

The presence of the phosphates in a deposit generally implies a neutral or alkaline state of the urine. If such be the case, and the presence of uric acid implies an acid state from free acid, it would follow that uric acid and the phosphates must exist very rarely in the same deposit. When the calculus has consisted chiefly of the phosphates, I have not once found uric acid to exist with it. When traces of this acid have been present, careful examination showed it was in combination with some base. And when the calculus consisted chiefly of uric acid, the small ash which sometimes remains will rarely be found to consist of the phosphates.

In the above Table the phosphates occur 139 times. Hence 139 times in 450 the urine must have been neutral or alkaline to test paper.

If then phosphates indicate neutrality or alkalescence, and uric acid indicates free acid in the urine, we may conclude that the deposit of oxalate of lime, as it occurs in the above Table with uric acid, with urate of ammonia and with the phosphates, is independent of acidity and alkalescence, and that its presence in a layer does not indicate any particular state of the secretion.

Now, as such layer implies a certain state of the urinary secretion, the 450 layers may be taken to represent 450 states of the urine. 139 of these were neutral or alkaline, as so many

times the phosphates are found to occur. 311 were feebly or strongly acid to test paper.

Of these 311, in 97 much free acid was passing from

the system, as so often layers of

uric acid occur. 38 but little free acid was thrown

out, as so often mixed layers of urate of ammonia and uric acid

appear in the Table. 117 no free acid passed, although

litmus was reddened. 59 the state of the secretion is un

known, the oxalate of lime not

offering any indication of it. Omitting these 59 oxalate of lime, there are then 117 states in which no free acid is passing from the system, and 135 in which little or much free acid was thrown out. From this it appears that in 252 cases of the uric acid diathesis, there were 187 in which no free acid was passing, and in these, alkalies would be of no benefit, so far as neutralising free acid in the urine is concerned : that is, in nearly every second case of the uric diathesis, there was but little if any free acid in the urine to be neutralised.

In only 97 cases out of 252 was there much free acid secreted, or only twice in five cases were alkalies very necessary to remove the acidity of the urine; though in other cases these medicines might have been beneficial in some other respect.

There are two points in my analysis which I have found difficult of determination, when only a small quantity of the calculus or layer could be spared.

The first, which is rare, is, to distinguish between

ses

a inixture of urate of ammonia with oxalate of lime, and urate of ammonia with urate of lime. The other is to recognise a mixture of uric acid with urate of ammonia. There is no difficulty in either case when a sufficient quantity of the calculus can be spared. The microscope, which in fresh sediments on the last question is most satisfactory, with the powder of calculi has afforded me no assistance. In 40 cases the phosphates or the phosphates and

carbonates form the last layer. In 7 cases the whole calculus consisted of fusible

deposit. In 5 cases the whole calculus consisted of phos

phate of ammonia and magnesia. In these 52 cases out of 233, the calculus might have been lessened by the injection of dilute acids, and in 12 out of these the whole calculus might have been removed. This supposes however that in all these cases the calculi were in the bladder, and not in the kidneys, on which there is no satisfactory historical evidence.

In addition to these 52 cases there are 6 calculi which consist entirely of urate of ammonia and fusible deposit, and 19 in which fusible and urate of ammonia form the outside layer. In these cases, most probably, any acid injection would dissolve the fusible and decompose the urate of ammonia, and thus disintegrate the calculus. So that altogether in 75 out of 233 a solvent might have assisted in the removal, although in 18 only out of 233, or

about 1 in 13, could the calculus have been entirely removed. For this Sir B. Brodie has shown dilute nitric acid sufficient. Perhaps at some future time lactic acid, which possesses a peculiar power of dissolving the phosphates, may be found even more rapidly efficacious.

APPENDIX.

SINCE the foregoing paper was written, I have been examining a small collection of between twenty and thirty calculi, chiefly removed by Mr. Cæsar Hawkins.

The first of these was a small calculus about the size of a large nut, which had been divided : the section showed a large nucleus with a few thin layers around it. The nucleus was dirty yellow, semitransparent, crystalline, irregularly radiated, and rather soft. The external layers were much harder, whiter, and less crystalline. The nucleus entirely disappeared with heat, giving a most disagreeable and peculiar smell; it dissolved with little difficulty in nitric acid with effervescence, and when evaporated afterwards to dryness, it left a black residue, which ammonia did not alter. A little of the powder from the nucleus was boiled with water in a test tube : to this a drop or two of a solution of acetate of lead was added, and then an excess of caustic potash.

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