This explains to us why in acute rheumatism, when acid abounds, and there are copious sour-smelling perspiration, the urine is always loaded with lithates; whereas in typhus, when the powers of life are low, and free alkali is liable to be secreted by the kidney, the urine may be very scanty and very deep colored, and yet there is no deposit till some acid be added, when the whole becomes turbid. Such urine oftentimes appears slightly acid to test•paper, and it would appear that the lithates are secreted in the soluble form with excess of alkali, and that the affinity of the acid present is too weak subsequently to convert them into an insoluble form. The fact is certain, the explanation perhaps un satisfactory.; but it is the only one which our chemical knowledge of these salts at present gives.
When acid is formed in excess in the stomach in dyspepsia, and afterwards passes off by the kidney, it tends to check the flow of urine, causing a deficiency of water, and at the same time it determines the formation of the less soluble lithates, which the small quantity of water present cannot hold in solution when cold. To speak therefore of an excess of lithates is a fallacy, because their deposition may depend merely on the proportion of water, and the presence of free acid. When lithic acid is really in excess, it is more likely to occur in a crystalline form, uncombined with any base ' • and to this the name of the " lithio acid diathesis" more properly belongs than to that in which the deposit is amorphous. The only valid proof of an excess of the amorphous lithates would be that the whole quantity of lithic acid passed during the twenty-four hours was ascertained to be beyond the average of health.
To speak of an excess of earthy phosphates is a more complete fallacy than that just alluded to. They are so very soluble when free acid is present, so insoluble when free alkali is present, that such a deposit indicates nothing more than the fact of the urine being alkaline. It is true, that as happens with the lithates in typhus, the urine may have a slightly acid reaction to test-paper, and yet the ordinary chemical change may not take place ; in such cases the earthy phosphates may be deposited; and the only explanation that can be given is that they have been secreted in an alkaline condition, but the acid present is too weak to alter their chemical relations; for a single drop of stronger acid at ontse dissolves the deposit. Valuable information might no doubt be obtained from a knowledge whether the phosphoric acid be really in excess ; but this can only be ascertained by a quantita tive analysis, which requires much chemical skill and much ex penditure of time. There is always enough phosphoric acid present in the urine to form a deposit when converted into the insoluble phosphate of lime.
A deposit of earthy phosphates then only shows that alkali of some sort is in excess. Of this there are three principal causes: the decomposition of urea yielding free ammonia; the ingestion of alkalies or decomposable alkaline salts ; and the secretion of excess of ammonia by the kidney. The first of these, when the urine is fetid, is very generally associated with cystitis, and is also developed in a very short space of time in urine which was alkaline on emission ; the second is only a casual occurrence, which has no pathological value, and is only to be borne in mind as one of the possible causes ; the third is that to which the name of the phosphatic diathesis has been given. It is evidently con nected with states of debility, especially with exhaustion of nerv ous energy: we do not expect to find it always present, because of the constant daily variations in the acidity of the urine, but its recurrence at certain periods may aid us in ascertaining its specific causes. It should be remembered, too, that the amount of acid in the stomach at any given period is generally in an inverse proportion to that in the urine, and I have seen this most strikingly exemplified in cases of sarcina ventriouli when the fermentation going on in the stomach produced the greatest possible degree of alkalesoence. Closely related to this change is one in which, without fetor or absolute decomposition, the urea is. converted into the carbonate of ammonia, a change which is hastened by boiling, and gives rise to effervescence on the addition of acid.
In general terms, speaking of acidity and alkalescence of urine, we find them associated with very opposite conditions of health, modified by the actual state of the stomach at the period when the fluid is secreted. A man of full habit, who indulges in the pleasures of the table, and is not disposed to overtax his mental powers or his nervous system, is very likely to exhibit in his urine copious deposits of the lithate of ammonia, especially at those times when he has been suffering from acidity of stomach, and the acid has begun to pass off by the kidney. Whereas a man of spare habit and nervous temperament, during the period of exhaustion following any excitement either of brain or nerve, is very liable to phosphatic deposits, especially while the acid is stS1 'in the stomach, and before it has begun to pass off by the kidney. On the other hand, the urine of one whose digestive organs are in an enfeebled state will contain the one deposit or the other, according to the period after food at which it is examined.