Many and peculiar are the changes which occur in mixtures containing salts of bismuth. Bismuth subnitrate is the most frequently prescribed salt, being oftener used than all the other salts put together; then come bismuth carbonate and solution of bismuth in about equal proportions, and far behind them the salicylate and other special salts. It has already been observed (page 65) that bismuth subnitrate (BiON03.H20) parts with nitric acid when shaken or washed with water. On shaking bismuth subnitrate 3ij. with water for a few seconds it will be found that the water has become quite acid. It is the acid thus liberated which is active in forming the clot in bismuth-and-tragacanth mixtures; and we find that if the subnitrate be washed with hot water in which a little sodium bicarbonate has been dissolved, it can be easily diffused through the liquid without forming a clot. Silicic acid in trace exists in bismuth salts and plays a part in clotting.1 The following are examples of mixtures that clot:

No. I. made a very curdy mixture, rendered worse by trituration in a mortar, and after several days became so thick that the bismuth would not subside. No. II. also rapidly clotted. There are three ways of obviating the difficulty: (1) By making the mixture of bismuth subnitrate and water feebly alkaline before adding the tragacanth, as by the addition of a few drops of ammonia solution or a grain or two of sodium bicarbonate; (2) by the substitution of bismuth sub-carbonate for subnitrate; or (3) by the substitution of acacia for tragacanth in the pulv. tragac. co., which is the best alternative to adopt. This is well exemplified in the following:

1 In recent years dispensers have had less trouble and more uniform results with bismuth salts, as manufacturers now turn them out very wel washed and with less variation in density. It is also possible to buy them much lighter than formerly.

No. III. was first compounded with pulv. tragacanth. co. 3j., and after standing a little the bismuth formed a jelly-like clot with the gum which would not diffuse throughout the mixture. Tragacanth mucilage behaved similarly. A few drops of ammonia solution prevents the gelatinisation, but is inadmissible in the presence of strychnine.1 The prescription was therefore altered as shown in No. IV., and, dispensed secundum artem (bismuth and liquor in half the water, the powders with the rest, and the two mixed), clotting was obviated. Fresh acacia mucilage does not give a clotted mixture. It must be added gradually to the bismuth salt already mixed with water or infusion. As to adding mucilage to bismuth mixtures when not ordered, see page 40.

Bismuth Subnitrate and a Bicarbonate react according to the following equation:

2BiON03 + 2NaHC03 = Bi202C03 + 2NaN03 + H20 + C02.

The reaction is sometimes slow, but if it does not occur in dispensing the prescription, it is apt to take place after the mixture is sent out. Potassium bicarbonate acts more rapidly.

1 With the above quantity of liq. strychninae, there is, of course, no precipitate of strychnine in alkaline solution. The solubility of strychnine is 1 in 6,400 of water, but 10 minims of liquor per oz. can safely be dispensed without precipitation in a Sodium-bicarbonate mixture(see page 291).

On one occasion, probably on a summer day, this burst the bottle within half an hour; on another occasion, when the temperature was only 6o° F., it was dispensed, the bottle securely corked, laid on its side, and agitated at intervals in expectation of an explosion. The result was disappointing, for after twenty-four hours had elapsed it was still intact, and the internal pressure was not sufficient to blow the cork out even when partly released. It was treated as it might be at a patient's house, for several days, agitating occasionally, and removing the cork, but the effervescence was not enough to cause inconvenience, In dispensing such prescriptions the possibility of an explosion must, however, be taken into consideration. Some dispensers substitute bismuth subcarbonate for subnitrate; others object on principle to 'substitution,' so mix the subnitrate and bicarbonate with boiling water to hasten the decomposition. The result is practically the same in both cases, for the finished mixture in either case contains bismuth subcarbonate, and that which has been made up with bismuth subcarbonate contains also the full amount of free alkaline bicarbonate.

The dispenser should be guided by the following considerations: When, as frequently happens, aromatic spirit of ammonia is also an ingredient in the prescription, subnitrate may be used, because the ammonium hydrate will either convert the subnitrate into bismuth hydroxide, or else will absorb any carbonic-acid gas that may be produced. When the amount ordered of each ingredient does not exceed 7 or 8 grains to the ounce in the case of sodium bicarbonate, and 5 grains in the case of potassium bicarbonate, there is practically no chance of explosion. If these quantities are largely exceeded, add the bicarbonate to most of the water, and shake until no more will dissolve; then add the subnitrate rubbed down with the remainder of the water, shake up, and loosen the cork occasionally. Then send out with the message, ' This bottle must not be laid on its side; loosen the cork immediately .it is received.'Lastly, when no aromatic spirit of ammonia is ordered, and the ingredients are present in large proportion (over 10 grains of each to the ounce), and the medicine has to be packed up at once, so that the cork cannot be loosened for a long time, it is wiser to complete the reaction by heating than to incur any risk of explosion.