MAG 



MAG 



But Mr. Kir wan gives a different 

 result. 



In crystals. 1>y. 



Sulphuric acid . . 29.35 63.32 



Magnesia 17.00 36.68 



Water 53.65 



100 100 



Sulphate of magnesia is formed by pass- 

 ing sulphurous acid through water, in 

 which magnesia is diffused. At first it is 

 in a state of powder, which is gradually 

 dissolved, and by exposure to the air, it 

 deposits crystals, and passes into sulphate 

 of magnesia. It consists of 



Sulphurous acid 39 



Magnesia 16 



Water 45 



100 



Carbonate of magnesia, or the magnesia 

 alba, of the physicians, is a very import- 

 ant compound. The manufacture of 

 this on the large scale is thus conducted. 

 Instead of the pure sulphate of magnesia, 

 the bittern, or liquor remaining after the 

 crystallization of sea salt is used, and the 

 magnesia is precipitated by carbonate of 

 potash. When properly prepared it 

 is perfectly white, nearly or vvholh/ 

 tasteless, and very sparingly soluble in 

 water. The magnesia of commerce is 

 composed of 



Fourcroy. Kirwan. 

 Carbonic acid .... 48 34 



Magnesia 40 45 



Water 12 21 



100 100 



When common carbonate of magnesia 

 is exposed to a moderate heat, it is de- 

 composed : its carbonic acid disengaged. 

 It loses about half its weight, and the 

 magnesia remains nearly pure. 



Under the magnesian genus of fossils 

 are comprehended, not only those in 

 which magnesia is the ingredient which is 

 present in largest proportion, but those 

 also in which, though in a smaller propor- 

 tion, there exist the characters in some 

 measure peculiar to this genus. These 

 are softness, unctuosity, and being in ge- 

 neral destitute of hardness, lustre, and 



transparency, which are conspicuous in 

 many of those which belong to the sili- 

 cious and argillaceous genera. Magne- 

 sian fossils have usually a green colour, 

 more or less deep. 



MAGNET. See MAGNETISM. 



MAGNETISM, is supposed to have 

 been first rendered useful about the end 

 of the twelfth, <>r at least very early in the 

 thirteenth century, by John de Gioja, a 

 handicraft of Naples, who noticed the pe- 

 culiar attraction of metals, iron in parti- 

 cular, towards certain masses of rude 

 ore ; the touch of which communicated 

 to other substances of a ferruginous na- 

 ture, especially iron or steel bars, the 

 same property of attraction : these touch- 

 ed bars he observed to have a peculiar 

 and similar tendency towards one parti- 

 cular point ; that when suspended in 

 equilibrio, by means of threads around 

 their centres, they invariably indicated 

 the same point ; and that, when placed in 

 a row, however adversely directed, they 

 soon disposed themselves in perfectly 

 parallel order. In this instance, he im- 

 proved upon the property long known to, 

 but not comprehended or applied to use 

 by the ancients, who considered the load- 

 stone simply as a rude species of iron ore, 

 and curious only so far as it might serve 

 to amuse. Gioja being possessed of a 

 quick understanding, and of a strong 

 mind, was not long in further ascertain- 

 ing the more sensible purposes to which 

 the magnet might be appropriated. He 

 accordingly fixed various magnets upon 

 pivots, supporting their centres in such 

 manner as allowed the bars to traverse 

 freely. Finding that, however situated 

 within the reach of observation and com- 

 parison, they all had the same tendency, 

 he naturally concluded them to be go- 

 verned by some attraction, which might 

 be ultimately ascertained and acted up- 

 on. He therefore removed into various 

 parts of Italy, to satisfy himself whether 

 or not the extraordinary impulse which 

 agitated these bars that had been mag- 

 netised by friction, existed only in the vi- 

 cinity of Naples, or was general. The 

 result of his researches appears to be, 

 that the influence was general, but that 

 the magnets were rendered extremely 

 variable, and fluctuated much, when near 

 large masses of iron. The experiments 

 of Gioja gave birth to many others, and 

 at length to a trial of the magnetic in- 

 fluence on the surface of the water. To 

 establish this, a vessel was moored out at 

 sea, in a direction corresponding with 

 that of the magnet ; and a boat, having a 

 magnet equipoised on a pivot at its ceu- 



