48 



lated as store for future growth, and returned from its reservoirs into 

 the circulation, it may be difficult clearly to discover anything cer- 

 tain ; 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 observa- 

 tions, showing how far the durability of the heart wood depends on 

 the period at which a tree is felled. 



On the Manufacture of the Sulphate of Magnesia at Monte della 

 Guardia, near Genoa. By H. Holland, M.D. F.R.S. Read June 

 13, 1816. [Phil. Trans. 1816,^. 294.] 



The site of this manufactory is about eight miles N.W. of Genoa, 

 at about 1 600 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, as- 

 bestus, 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 ap- 

 pears 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, to- 

 gether with a certain portion of magnesian limestone, are the mate- 

 rials used in the manufacture of the sulphate of magnesia, in an es- 

 tablishment 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 acci- 

 dental product, but has now become the principal object of the work. 

 For this purpose the pyrites is extracted from the mountain by tun- 

 nels, 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 

 ,-Hth 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 



