in ammoniacal gas and hydrogen, the mercury assuming its ordinary form. Phosphate of ammonia is very valuable from its property of rendering the lightest materials, such as gauze or muslin, incombustible. Dip a piece of muslin in a solution of phosphate of ammonia, and dry it in contact with the air; that done, you will find it is impossible to set fire to the material; it will get charred, but you cannot make it burn. It is to be wished that this useful precaution were oftener taken in the matter of ball-dresses, which have so frequently been the cause of serious accidents. There is no danger whatever with a dress that has been soaked in phosphate of ammonia, which is very inexpensive, and easily procured. [Illustration: Fig. 302.—Experiment with ammonium.] For preparing cool drinks in the summer ammoniacal salts are very useful; some _nitrate of ammonia_ mixed with its weight in water, produces a considerable lowering of the temperature, and is very useful for making ice. _Volatile alkali_, which is so useful an application for stings from insects, is a solution of ammoniacal gas in water, and _sal-volatile_, which has such a refreshing and reanimating odour, is a carbonate of ammonia. We often see in chemists’ shops large glass jars, the insides of which are covered with beautiful transparent white crystals, which are formed over a red powder placed at the bottom of the vase. These crystals are the result of a combination of cyanogen and iodine. Nothing is easier than the preparation of _iodide of cyanogen_, a very volatile body, which possesses a strong tendency to assume a definite crystalline form. We pound in a mortar a mixture of 50 grams of cyanide of mercury, and 100 grams of iodine; under the action of the pestle the powder, which was at first a brownish colour, assumes a shade of bright vermilion red. The cyanogen combines with the iodine, and transforms itself into fumes with great rapidity. If the powder is placed at the bottom of a stoppered glass jar, the fumes of the iodide of cyanogen immediately condense, thereby producing beautiful crystals which often attain considerable size (fig. 303). Cyanogen forms with sulphur a remarkable substance, _sulpho-cyanogen_, the properties of which we cannot describe without exceeding the limits of our present treatise; we shall therefore confine ourselves to pointing out one of its combinations, which is well known at the present day, owing to its singular properties. This is sulpho-cyanide of mercury, with which small combustible cones are made, generally designated by the pompous title of _Pharaoh’s serpents_. For making these, some sulpho-cyanide of potassium is poured into a solution of nitrate acid on mercury, which forms a precipitate of sulpho-cyanide of mercury. This is a white, combustible powder, which after passing through a filter, should be transformed into a stiff pulp by means of water containing a solution of gum. The pulp is afterwards mixed with