If the solarization be very deep, apply the lamp beneath and slightly warm the plate, pour suddenly off, and without rinsing, quickly apply the gilding and gild in the usual way. The whole operation must be quickly performed, or the chlorine soon attracts the shade of the picture. There may be instances when this solution may prove of advantage, as, for instance, when black velvet and milk white are wanted in the same impression. We have seen it operate with pleasing success; but repeat that _few_ only use it, yet as in a single instance it might be of value, we have given it. As much of the muriate of ammonia, in common, is adulterated, we will give a few tests which all can try. When pure, this salt is totally volatilizable by heat; if a small portion on being heated on a piece of platinum foil over a spirit lamp leaves any fixed residue, it is adulterated. It should dissolve entirely in water; if it leaves an _insoluble residue_ it is adulterated. The impurities generally found in sal-ammoniac, are sulphate of ammonia, sulphate of soda, chloride of sodium, and chloride of potassium; neither of these are considered injurious in the _bleaching solution_. Occasionally it contains lead, iron, and copper. The presence of sulphuric acid may be detected by means of a diluted solution of _chloride_ of barium, added to a weak solution of muriate of ammonia, in water, this will produce a white precipitate, which is the more copious and dense, the less the liquor is diluted. If it contains lead add to a small portion of it, contained in a test tube, a little diluted _sulphuric acid_, or a few drops of a solution of _sulphate of soda_ (glauber's salt), if the liquid contains lead there will be prevalent a white powder, or precipitate, this powder scarcely dissolves at all in diluted acids, but it dissolves in a solution of caustic of potash: _iron_, mix a solution of the yellow prussiate of potash with a solution of red prussiate, a few drops of this added to a weak solution of muriate of ammonia will produce a blue precipitate; _copper_, pour about half an ounce of the liquid in a test tube, and add to it a few drops of liquid ammonia, if copper is present, the liquid will assume a blue color. Sal-ammonia is known by giving out the odor of ammonia when mixed with caustic of potash: when sal-ammonia possesses a _brownish_ color, it indicates the presence of charcoal, or empyreumatic oil; such sal-ammoniac is good for some purposes, but wholly unfit for _chemical purposes_. When sal-ammoniac leaves a non-volatile residue, it may contain _sulphate of soda_. This is the principal cause of failures with the bleaching solution, as the sulphate of soda has a tendency to blacken rather than bleach Daguerreian impressions. The sulphate of soda as well as the chloride of sodium, is often found in unpurified sal-ammoniac to the amount of ten per cent.