salt indispensable to the integrity of the blood and body. But they take phosphate of potash, and a salt of soda, chloride of sodium. In these animals, however, we find in the blood phosphate of soda, and in the muscles chloride of potassium; neither of which salts they have taken with their food. From this fact, and from other experiments in the laboratory, the conclusion is inevitable, that phosphate of potash, when mixed with chloride of sodium, gives a part of its potassium off to some chlorine, which in its turn parts with some sodium. The latter combining with the oxygen liberated from the potassium, joins the phosphoric acid, phosphate of soda being formed.

The relations of the salts of potash to the muscles are not as yet understood. But the same physiological law which confines phosphate of soda to the blood, and makes it indispensable there, is, no doubt, active in confining the salts of potash to the muscles. To bring about an understanding of these relations is a problem of experimental physiology.

CHAPTER XIII.

LIME AND MAGNESIA.

Symbols: CaO and MgO.

Lime and magnesia occur in the urine in combination with phosphoric acid, as acid phosphates in solution. They are met with as deposits in alkaline urine, and as concretions in urinary calculi, of which they most frequently constitute the crust.

Formula: PO,+2CaO+HO in solution.

POS+2MgO+HO

}

Some authors calculate the formula for phosphate of lime and magnesia so that the three equivalents of water of phosphoric acid are replaced by three equivalents of the earths, PO,+3CaO, and PO,+3MgO. These formulæ are transferred from the formulæ of bone-earth, obtained by incineration of the bones. But as incineration of phosphates in the presence of carbon always causes a loss of phosphoric acid, these analyses cannot be depended upon as regards the composition of the phosphates. Moreover, the composition of the triple phosphate (of magnesia) militates against the basic formula. The precipitates, however, obtained from urine by the addition of caustic fixed alkalies, both show the composition of three of oxyde to one of acid, the magnesia salt retaining five equivalents of water after drying at the heat of boiling water.

Aggregation.-As generally met with, both phosphates of lime and magnesia are amorphous. But they are obtained crystalline, the one from certain solutions, the other in combination with a third equivalent of base and water of crystallization.

When a solution of phosphate of lime in acetic acid is allowed to stand some time, the phosphate has a great inclination to fall down from this solution in a crystalline state, particularly when the mixture is warmed a little, and when the phosphate is prevalent.

Separation of Phosphate of Lime and Magnesia from Urine, and determination of their collective amount.

This is simply effected by adding to urine some ammonia, until a strong alkaline reaction is observed. The earthy phosphates thus precipitated may be collected on a filter, washed, exposed to red heat, and weighed. Or, after collection on the filter, they may be redissolved in acetic acid, and in this solution the amount of phosphoric acid, found by the analysis described above, may, by the equivalents known, indicate the amount of earths in combination. Or, if the phosphates of the alkalies are to be determined also, the analysis described at p. 185 may be used, by which, in the first instance, the entire amount of phosphoric acid contained in the urine, and then of phosphoric acid in combination with the alkalies, is determined, the difference corresponding to the amount of phosphoric acid in combination with the earths.

Modes of ascertaining the separate quantities of Lime and Magnesia in Urine.

All the following methods are preceded by the separation from urine of both earths by means of ammonia, and by the solution of the washed precipitate in acetic acid. From the latter solution the lime is always obtained as the oxalate. From this point the analyses begin to vary.

Determination of the Lime as Sulphate.

The oxalate of lime obtained by addition of oxalate of ammonia to the solution in acetic acid of the precipitate, which a measured quantity of urine yielded after addition of ammonia, is exposed to a strong red heat in a platinum cap

sule. To the lime thus transformed into carbonate, partly into caustic lime, some sulphuric acid is added, with great care, in small portions, by means of an elastic pipette, until an acid reaction is observed. After drying, and a second exposure to red heat, the lime is obtained in the form of sulphate, which, by multiplication with C-4118, gives the amount of lime contained in the sulphate.

Determination of the Magnesia as Pyrophosphate.

The filtered fluid obtained from the oxalate of lime in the course of the foregoing analysis, is treated with ammonia until alkaline, whereby the magnesia is precipitated in the form of PO+2MgO+NH,O+12Aq, triple phosphate, or ammonio-phosphate of magnesia. As the crystals settle easily to the bottom of the vessel, this salt may be washed by decantation of repeated quantities of water, containing some ammonia, added to it. It is then collected in the platinum capsule, and exposed to red heat. If collected on a filter, the latter, after removal of the greater bulk of the crystals into the platinum capsule, has to be burned separately by Bunsen's process, described in the introduction. Precipitate and filter, when united, are then exposed to red and white heat. The triple phosphate, by losing all its water and ammonia, is thus transformed into pyrophosphate of magnesia, of the composition PO,+2MgO. The amount 40.000 of the latter, multiplied by = 0·3592 (111·360 111.360 equivalent of pyrophosphate of magnesia, containing 40.000

of magnesia)-more accurately

41.302 112.738

= 0.3687, gives the

amount of magnesia.

Hempel's method of determining the Lime as Oxalate, by means of Permanganate of Potash.

From the solution of the mixed carths in acetic acid the lime is precipitated by oxalate of ammonia, collected on a filter, and, after washing, is dissolved in a few drops of hydrochloric acid. To this solution, after warming it a little, a graduated solution of permanganate of potash is added, so long as discoloration continues to take place. From the amount of solution of permanganate of potash used, the amount of oxalic acid is found, and from this the amount of lime originally combined with phosphoric acid.

Preparation of the solution of permanganate of potash.As the solution of permanganate of potash cannot be kept as such for any length of time without undergoing changes, its graduation has to be checked before each analysis. For this purpose, and for the original graduation, a solution of oxalic acid in water is used, which in every cubic centimètre contains 10 milligrammes of oxalic acid. The solution of permanganate of potash is then graduated so that 10 c.c. will exactly suffice to oxydize 10 milligrammes of oxalic acid, and to make the signal of the completed oxydation by the appearance of the red colour.

The solution of oxalic acid is simply made by dissolving one gramme of the acid, dried at 212° F., in 100 c.c. of water. Of this solution 10 c.c. are taken by means of a pipette, and transferred to a beaker. We now add, from a burette, of the solution of permanganate of potash which is to be graduated, as much as necessary to produce the lasting red test. Supposing that 83 c.c. have been used for that purpose, we then add to every 830 c.c. of the solution, 170 c.c. of water, whereby we obtain 1000 c.c. of solution, of which 10 c.c. will exactly suffice for oxydizing 10 c.c. of the solution of oxalic acid, containing 100 milligrammes of the acid.

It is, however, less troublesome to merely determine the quantity of solution of permanganate of potash required for oxydizing a known amount of oxalic acid, without adjusting its bulk to that of the solution of the acid.

If, therefore, the above solution be taken as it is, without adding the 170 c.c. of water, then its value would be expressed by 8.3 c.c. 100 milligrammes of oxalic acid. The calculation after the use for real experiment would therefore be 8.3 c.c. of standard solution: 100 milligrammes of oxalic acid =nc.c. standard solution: a milligrammes of oxalic acid.

This analysis has been combined by Professor Vogel with the volumetrical analysis of phosphoric acid, so that by two analyses the quantities of phosphoric acid and both earths are determined. He for that purpose takes the precipitates from two equal quantities of urine, and determines in the one the entire amount of phosphoric acid, in the other the amount of lime as just described. From the total amount of phosphoric acid the equivalent for lime is now deducted; the rest of the acid indicates the amount of magnesia with which it was combined.

The operator must take care to use a Gay-Lussac's burette for the solution of permanganate of potash, as this fluid is decomposed by the India rubber of Mohr's burette.