The mode of obtaining fluoric acid for chemical purposes is, by pouring sulphuric acid upon powdered spar in a leaden retort, and applying to it a gentle heat. This acid should be used with great caution; for, when applied to the skin, it instantly disorganizes it, and produces very painful sores. 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.