Page:Proceedings of the Royal Society of London Vol 2.djvu/80

Rh as store for future growth, and returned from its reservoirs into the circulation, it may be difficult clearly to discover anything certain; but the author has not ceased to prosecute his experiments on the varying density of the alburnum, and other parts of the wood, and on the proportion of moisture which they lose by drying; and he hopes at some future time to lay before the Society his observations, showing how far the durability of the heart wood depends on the period at which a tree is felled.

The site of this manufactory is about eight miles N.W. of Genoa, at about 1600 feet above the level of the sea, from which the top of the mountain is five miles distant, and elevated about 2000 feet. The ascent from Sestri is by a deep ravine, the course of a torrent, the eastern side of which is composed of serpentine in vast masses, lying unconformably on primitive schist, and containing talc, steatite, asbestus, and many small veins of pyrites. On the western side of the ravine are mountains of magnesian limestone. In passing to the upper end of this ravine, the stratification of the primitive schist appears mixed with chlorite, slate, and other magnesian minerals, and containing numerous veins or layers of pyrites, both of copper and iron. The substance of these ores is schistose, as well as the rock in which they lie, and they are so intimately mixed with the same magnesian minerals, as to feel unctuous to the touch. These, together with a certain portion of magnesian limestone, are the materials used in the manufacture of the sulphate of magnesia, in an establishment originally set up for converting copper and iron pyrites into sulphates of those metals.

The sulphate of magnesia was at first observed only as an accidental product, but has now become the principal object of the work. For this purpose the pyrites is extracted from the mountain by tunnels, the largest of which is about 200 feet in length, and from 10 to 15 feet wide. The ore is then broken into small pieces, roasted for about ten days, and being then collected in heaps, is kept moist with water for several months, during which the salts are forming. The materials are then lixiviated, and after the liquor has been filtered through sand, the copper is first precipitated by refuse iron, after which a portion of lime, prepared from the magnesian limestone of the adjacent mountain, is added, in order to precipitate the iron, and at the same time to make some addition to the product of sulphate of magnesia.

The circumstance particularly to be attended to in this process, is the proportion of lime employed, which in general does not exceed th of the weight of ore. For if this were added in excess, it would occasion the precipitation of the magnesia along with the metals. The whole produce of this manufactory, we are told, does