BARYTES FAMILY.
195. These minerals are sometimes called ponderous earths, and have their name from a Greek word signifying heavy. They comprehend all the combinations of barytes with acids.
When purified, they form a greyish white, porous substance, which is easily reducible to powder; has no perceptible smell, but has a harsh and more burning taste than lime, and changes the blues of vegetable colours to green.
Although barytes is one of the most useful chemical tests that we are acquainted with, it is not much employed in the arts, because, when purified, it is found too expensive. It is capable of being made into a very tenacious cement; and painters use a preparation that is made from it as a white colour which will not change. This is sold in the shops under the name of “Hume’s permanent white.” Barytes taken into the stomach proves a virulent poison; yet a preparation of it is used in medicine, and particularly for the removal of scrophulous complaints. When finely pounded and mixed with oatmeal, carbonat of barytes has been found an efficacious poison for rats.
196. SULPHAT of BARYTES is a mineral formed by the combination of sulphuric acid ([24]) with barytes.
It sometimes occurs in a state of powder, frequently in shapeless masses, and often crystallized: the primitive form of its crystals being a four-sided prism. It is not soluble in any other than sulphuric acid.
With us sulphat of barytes is of no use in the arts. The Chinese, however, employ it as an ingredient in the composition of porcelain; and it is said to form a good manure for clover fields.
The Bologna Phosphorus, or Bononian Stone, a very remarkable kind of barytes, has its name from being found near Bologna in Italy. This substance, when detached, is usually observed in roundish, flat, kidney-shaped pieces, from about the size of a walnut to that of an orange, which have a shining and somewhat fibrous texture within.
When the outer coat of this stone is washed away by heavy rains, it has sometimes the appearance of burnished silver. An Italian shoemaker, in the year 1630, deceived by this appearance, carried home several pieces, hoping, by means of fire, to extract silver from them. But at the same time that he was disappointed in this expectation, he was surprised by a very unlooked-for phenomenon. All the pieces which he had thus attempted to melt, when they were afterwards exposed to the light, became themselves luminous. It is the singular property of the Bologna phosphorus, after it has undergone calcination in a particular manner, to become capable of imbibing so much light on exposure, for a little while, to the light of the sun, or even to the flame of a candle, that it will afterwards shine in the dark for an interval of from eight to fifteen minutes, like a glowing coal, but without any sensible heat. The light which it emits is sufficient to read by, provided the letters be placed near it. If well prepared, the stone will retain this extraordinary property for five or six years.
The preparation of it is thus conducted. Pieces of sulphat of barytes are made red hot, for a few minutes, in a covered crucible placed in the middle of a fire, and then left to cool. When cool, they are pounded in a stone mortar, and sifted. The powder thus formed is made into a paste with a little gum arabic, and divided into long cakes, or cylinders, each about a quarter of an inch thick. These pieces are dried in a moderate heat, and then, by degrees, are exposed to a more violent heat, among charcoal, in a wind furnace. As soon as the coals of the furnace are half consumed, it must be filled a second time, and the phosphorus must be left undisturbed. When the coals are quite consumed the ashes must be carefully blown off with a pair of bellows, and the phosphorus will be found at the bottom of the grate.