Preparation and Composition. in the preparation of this salt, bones are first calcined, and then treated with sulphuric acid and a little water. The acid takes the greater part of the lime of the phosphate of lime of the bones, forming insoluble sulphate of lime, and leaving the remainder combined with a great excess of phosphoric acid, in the state of a soluble superphosphate of lime, which is dissolved out of the mass by boiling water. To the solution of superphosphate of lime thus obtained, after due concentration, a hot solution of carbonate of soda is gradually added, so long as effervescence is produced. The carbonic acid of the carbonate escapes, and the soda, combining with the excess of phosphoric acid, forms a soluble phosphate of soda; while the neutral phosphate of lime to which the superphosphate has been reduced, being insoluble, is precipitated. The liquor being filtered while hot, deposits, on cooling, the phosphate of soda in crystals. The salt obtained is the tribasic phosphate, consisting of one equivalent of acid, two of soda, one of basic water, and twenty-four of water of crystallization.

Properties

Phosphate of soda, recently crystallized, is in large, finely transparent, oblique, rhombic prisms, which, however, rapidly effloresce on exposure, and become white and opaque. it is inodorous, and has a purely saline, not disagreeable taste, very similar to that of common salt, though much feebler. It is soluble in four parts of cold and two of boiling water, and is nearly insoluble in alcohol. in consequence of its efflorescence, it gradually becomes stronger, in the same dose, if exposed to the air; and, as the water of crystallization amounts to nearly two-thirds of the weight of the crystals, it is obvious that the degree to which the efflorescence has proceeded should be considered in regulating the dose. When heated moderately, the crystals melt, and give out their water of crystallization; and, if the heat be pushed to redness, part also with their basic water, being converted into a white mass, which is the pyrophosphate of soda. When thus dried, the salt loses 62.3 per cent. of water.

Incompatibles. Phosphate of soda is incompatible with the soluble salts of lime, and with most of the neutral metallic salts, forming insoluble phosphates with the bases.

Medical Properties and Uses

This salt was first introduced into use as a medicine by Dr. George Pearson, of London, about the beginning of the present century. it has all the general properties of the saline cathartics, though less efficient than some of them. its chief recommendation is its simple saline taste, which enables it to be taken, when the other salts might be rejected, and even to be administered to a patient without his own knowledge,, by substituting it for common salt. The condition, then, under which it is specially indicated, is the existence of an insuperable aversion of the patient, or an obstinate resistance of the stomach to the ordinary saline cathartics, when these are called for by the symptoms.

Possessing, in its phosphoric acid, a material essential to the constitution of the system, it has been theoretically supposed that it might be advantageously used in affections characterized by a deficiency of that material, such as mollities ossium, rickets, disordered nutrition with deficiency of the phosphates in the urine, and scrofulous and tuberculous disease, in which a similar deficiency is supposed to exist in the tissues. But, when it is considered that, in all such cases, if the deficiency really exist in the organic structures, it is not from a want of a sufficient supply of the material, which is always abundantly offered in the food, but from a defective power of assimilation, it will be readily admitted, that the failure of experience to confirm these favourable expectations is nothing more than might have been anticipated. The probability is, that the phosphate of soda is capable of answering the purposes to which the saline cathartics generally arc adapted, and nothing more.

The dose of the salt is one or two ounces; the proportion of water being so great as to render the real amount of saline matter in the crystals relatively small, and the dose consequently larger than of most other medicines of the class. it is said that a dose of it may be given dissolved in a bowl of soup or gruel, and the patient not be aware that he is taking anything more than common salt with his food.