Hydrofluoric acid, diluted with water, is principally used in glass etching, but there are several fluorides used for the purpose. Fluoride of ammonia is formed by adding ammonia to hydrofluoric acid until it is nearly saturated; if a slight excess of ammonia is added so that the mixture smells of it, and then a little more acid be mixed with this, the fluoride will be suitable for glass etching. The fluoride of ammonia is placed on the glass and allowed to dry, when the etching effect then becomes apparent. Another etching fluid is made by dissolving 25 parts of fluoride of potassium, 25parts of hydrochloric acid, and 14 parts of sulphate of potash in 100 parts of water. Another solution is made by dissolving 10 parts of carbonate of soda and 10 parts of carbonate of potash in 40 parts of warm water, and then adding 20 parts of concentrated hydrofluoric acid and 10 parts of sulphate of potash previously dissolved in 10 parts of water.