Alum is obtained in a different manner at Solfaterra, near Puzzuola. lie re nature acts synthetically. Fumes pregnant with sulphurous and sulphuric acid, are con tinually issuing from little crevices in the volcanic soil of this place ; the former of which deposit a concrete sulphur; the second gradually penetrate the ancient la vas, which are of an argillaceous nature, combine with their alumine, and thus lbrm an alum ore, which after wards affords by lixiviation and crystallization a very pure alum.
In other varieties of alum ore the process is different. Those which are hard, and contain much pyrites or sul phuret of iron, require to be masted or calcined. Those which arc softer require merely exposure to air and hu midity. The ore is placed in large heaps, sloping to a ridge like the roof of a house, and left exposed to the open air for a year or more. When its decomposition, which is particularly promoted by damp air, is suffi ciently advanced, it is distributed into long flat troughs, and lixiviated. When the water is sufficiently satura ted with the salts, which arc sulphate of alumine and sulphate of iron, it is carried to the manufactory, and boiled in leaden cauldrons, till the proof liquor taken out becomes, on cooling, a crystalline mass of the con sistence of honey. During the long boiling of the lixi vium, the greater part of the sulphate of iron is de composed, the iron passes to a higher degree of oxyda tion, in which state so much of it cannot be dissolved in sulphuric acid, and it is deposited in the form of brown oxyde. When the lixivitnn is sufficiently boiled down, it is carried to the settling troughs, and as soon as it has grown clear by standing a little, it is drawn off into other troughs, where it is mixed with the quantity of potash necessary for making it into alum.
In the process which is followed at Whitby, the ore is laid in large heaps, and burned with wood or thorns, un til it be white ; or it is sometimes burned by rearing a pile of the ore, and of coal, in alternate layers, kindling it at the bottom. When it is sufficiently calcined, it is put into a pit, about ten feet long, six feet broad, and five feet deep, where it is macerated in water for eight or ten hours ; the ley is then drawn off into a large cistern, or reservoir, and a fresh quantity of water is put on the calcined ore in the pit. This after some time having acquired a sensible but weaker saline impregnation than the former, is likewise run off into the cistern. From this the layer is conveyed into pans, where it is kept. boiling for twenty-four hours. There is then added to it a ley prepared from kelp; and after some time, the li quor is drawn off into a vessel, where it may settle, or the impurities subside. When clear, it is run into
coolers, where it is allowed to remain about four days and nights, and a quantity of putrid urine, in the propor tion of about eight gallons to a ton of liquor, is added. The alum crystallizes ; the residual liquor is removed, and mixed with the liquor obtained from a fresh quan tity of materials which is to be evaporated. The crys tallized alum is washed, the water being allowed to drain from it : it is melted in a pan; it is, while liquid, con veyed into tuns, in which it is allowed to remain about ten days, so as to become fully concrete. The tuns are unhooped, and the large masses of alum broken down, and stored for the market.
In the prepaiation of alum at lord Dundas's works, in Yorkshire, inuriate of pota-,11 is added instead ol kelp. The proportion employed is about 41cwt. for every tun of alum produced.
Alum being thus obtained from these mineral sub stances, it is obviously an interesting subject to discover their composition, whether they contain it ready Formed, or only' its elements; and whether these are present in the pi °portions and combinations most favourable to its uction. This has accordingly engaged the attention of Kiain oth and Vauquelin.
Klaproth analysed the alum-stone of 'culla, and found that, in its natural slate, it contains the three essential constituent principles ni alum ; sulphuric acid, argil, and potash. Its ingredients and their proportions were 56.5, argil 19, sulphuric acid 16.5, potash 4, water 3. The same ingredients had been discovered in it by Vauquelin in proportions somewhat different. The aluin-earth ol Freienwald, that which affords alum by the third process above described, Klaproth found to be oi a very different composition. It appears to consist essen tially of sulphur and carbon, with argil, and saline sub stances having a base of potash. The results, as stated by this chemist, are, sulphur 28.5, carbon 196.5, argil 160, silex 400, oxide of iron, with a trace of manganese, 64, sulphate of iron 18, sulphate of lime 15, magnesia 2.5, sulphate or potash 15, muriate of potash 5, water 107.5. This substance is peculiar, as not containing the sulphur in combination with iron, but, as Klaproth supposes, combined with carbon. It becomes oxygeni zed from exposure to the air, and thus forms the sulphu ric acid which combines with the argil, and this receiv ing potash from the sulphate and muriate of potash, the alum is formed.