able pro phosphate There is no known salt, the chemical characters Remarkof which approach more closely to those of the perties of serum of blood, than the phosphate of soda; there of soda. is none more fitted for the absorption and entire removal from the organism of carbonic acid. This salt behaves towards carbonic acid exactly as neutral carbonate of soda; its aqueous solution absorbs carbonic acid gas with the same facility, but with It not only this difference, however, that under the influence of but also the same causes which decompose the neutral car- carbonic' bonate, and the bicarbonate of soda, this solution gives off the carbonic acid which it has absorbed lity. much more easily, and also more completely, since it does not, like soda, in its conversion from bicarbonate into neutral carbonate, retain any portion of carbonic acid. When carbonic acid gas is placed in contact with a solution of 1 part of dry phosphate of soda (PO,, 2 Na O, HO), in 100 parts of water, twice as much carbonic acid is absorbed as an equal volume of water, at the same temperature, can take up.* By simple agitation with air, or by diminution of the atmospheric pressure, 2-3rds of the absorbed carbonic acid are given off at the ordinary temperature; by contact with fresh carbonic acid, these 2-3rds are immediately again absorbed.† * A solution of phosphate of soda, saturated with carbonic acid, may be recommended as one of the pleasantest saline purgatives. absorbs, gives off, acid with great faci Experi † A solution of 1 part of dry phosphate of soda, P Os, 2 Na O, ments. Uses of the phosphate of soda in blood. By the spontaneous evaporation in the air of the solution of phosphate of soda, saturated with carbonic acid, the whole of the carbonic acid is given H O, in 100 parts of water, when agitated with pure carbonic acid gas, free from atmospheric air, absorbed : Solution, cubic centimetres Carbonic acid absorbed c. c. I. II. III. IV. 59 38 62 56 104 77 114 112 100 vols. of the solution absorb, therefore 176 203 183 200 Mean amount of gas absorbed by 100 vols. of solution = 190 vols. The water which had been used for the solution was treated in the same way, and absorbed : 85 98 100 vols. of water absorb, therefore 95 Mean amount of gas absorbed by 100 vols. of water = 92 vols. A portion of the solution of phosphate of soda, as above, was saturated with carbonic acid, and then agitated with repeated portions of air, as long as any carbonic acid was expelled. The solution was now placed in contact with pure carbonic acid gas, and absorbed : Solution, c. c. Carbonic acid absorbed c. c. I. II. III. IV. 62 67 68 89 88 91 99 116 100 vols. of solution absorb, therefore... 143 134 145 130 Mean amount absorbed by 100 vols. of solution = 130. off, and the phosphate is left, with all its original properties, including its alkaline reaction. When carbonic acid is taken up by the blood, there is established between the phosphoric and carbonic acids an equilibrium, similar to that existing in the juice of flesh between the phosphoric and lactic acids. In the same way as these last divide between them the potash of the juice, so do the carbonic and phosphoric acids divide between them the soda of the blood. There can be no circumstances more favourable to the separation of one or other of the two acids. If we assume, that the carbonic acid seizes a portion of the soda, we may imagine that the phosphoric acid, previously combined with this portion of base, is expelled from the place it originally occupied, and thus set free; but it does not yet, on that account, separate from the compound. We can say that the carbonic acid is converted into carbonate of soda, only when the free phosphoric acid has been removed, and employed in another quarter; but in point of fact, this phosphoric acid, thus displaced, is always present, and retains, unimpaired, its power of again combining with the soda. The slightest cause, coming in aid of its affinity, so as to give it the preponderance (and to this category belong all causes which diminish the affinity of carbonic acid for soda), suffices to displace the carbonic acid, and to reproduce the original compound. Agitation with air; the spontaneous evaporation of the water, in Action of carbonic acid on the blood. which the compound is dissolved; the diminution of the atmospheric pressure; all these causes, which have no effect on neutral carbonate of soda, produce decomposition, and cause the separation of the carbonic acid, taken up by the phosphate of soda in the blood. In this manner, the amount of carbonic acid in the blood is kept at a constant value. If more carbonic acid enter the blood from the body, more phosphoric acid is set free in proportion, and thereby a more easy and complete separation of the carbonic acid in the lungs is secured. If more soda be taken up, then a part of the carbonic acid, which would otherwise have escaped by the lungs and skin, is expelled by the urinary passage in the form of carbonate of soda. It is easy to foresee, that a more exact study of the influence which alkalies, salts, and mineral acids exert on the respiratory process in the normal state, must lead to the most beautiful and valuable results in regard to their employment in various diseases. It has already been pointed out, that in the juice of flesh the amount of phosphate of lime, compared with that of phosphate of magnesia, is very trifling. In fact, the juice of ox-flesh contains so little lime, that the quantity obtained from many pounds of flesh amounted only to a few milligrammes (1 milligramme 1-75th of a grain, nearly); but in the juice of the flesh of fowls, the relative proportions of these two bases admitted of more exact determination. The juice of fowl's flesh was precipitated by in the juice baryta, the precipitate dissolved in hydrochloric magnesia acid, the baryta separated by sulphuric acid, and of fowl. then the phosphoric acid removed by means of sesqui-chloride of iron and ammonia. The lime and magnesia then remained in solution. There were obtained 0.72 gm. of carbonate of lime, and 0.431 gm. of phosphate of ammonia and magnesia; or for 10 parts, by weight, of lime, 39.2 parts of magnesia. 59 of alkaline The proportion of the phosphoric acid combined Proportion with alkalies, to that united with magnesia, in the phosphates. juice of ox-flesh, was determined in the following manner. The precipitate formed by baryta contains all the phosphoric acid, partly combined with baryta (as PO,, 3 Ba O), partly with magnesia (as POS 3 Mg O). This precipitate was decomposed by sulphuric acid, and the liquid, filtered from the sulphate of baryta, was precipitated by ammonia. In this way the magnesia was thrown down, in the form of the usual double phosphate. The liquid filtered from this precipitate contained the phosphoric acid originally combined with alkalies, and when mixed with sulphate of magnesia yielded a new precipitate of the same double phosphate of ammonia and magnesia. The weight of the first precipitate was to that of the second as 0.2782 to 0.974, or as 1 to 3.5. For 2 atoms of phosphoric acid, therefore, combined with magnesia, the juice of ox-flesh contains 7 atoms of phosphoric acid, combined with alkalies, chiefly potash. In another experiment the proportion was found to be as 1 to 3.2. |