FARMERS' REGISTER— MAGNESIA. 



427 



undulations through tlie island, and has topsy-tur- 

 vied stronger brains than mine. 



1. The first remark I beg leave to make on this 

 gentleman's Essay is, that he does not point out 

 the stale or combinations of the magnesia, which 

 produces such dreadful effects. This question I 

 have repeatedly put to several of my friends, Avho 

 were visibly under the highest parox3'sm of tiie 

 magnesite-phobia ; but, whether from ignorance, 

 or from the disorder having reached their cranium, 

 I never could draw from them any distinct answer. 

 In one point only they all concurred, in expressing 

 the greatest horror and alarm at magnesia coming 

 within a hundred miles of their land; and some, 

 when they learnt that this deleterious substance 

 was capable of combinations, became still more 

 alarmed lest it might be smuggled in upon them 

 in a form which they had no means of detect- 

 ing. 



2 Your correspondent Tyro not only omits 

 specifying the state of the magnesia which proves 

 so hurtfid to land, viz. Whether it is pure carbo- 

 nate, or with what acid it is combined, but also 

 neglects to mention tiie mode by which the solu- 

 tion of the limestone is effected. All I am left to 

 gather on this point is, that the solution was effect- 

 ed in a way similar to that, the fallacy of which I 

 have already pointed out in a preceding essay. 



Whatever way his solution was effected, I can 

 only say, that his ammonia, or vol-alkali, would 

 throw down several other matters before it touch- 

 ed magnesia ; and that his whole sclienie of ana- 

 lysis appears liable to objections. I might enter 

 into particulars in order to establish this assertion ; 

 but, not wishing to embark in any controversies 

 remote from practical utility, I shall content my- 

 self with showing that magnesia, under certain 

 combinations, is either harmless or beneficial to 

 the growth of plants. 



3. It must appear strange to any man endowed 

 with reason, that magnesia, which is evolved by 

 the putrefaction and combustion of (I may venture 

 to say) all animal and vegetJibJe substances, should 

 operate as a poison to plants. How does this sub- 

 stance get into plants ? doul)tless by their roots. 

 How does it get into animals.^ doubtless by feed- 

 ing on plants which contain it.' How does it get 

 into animals which do not feed on plants.' doubt- 

 less by devouring animals which feed on plants. 

 The uses of magnesia in the vegetable and animal 

 economy are yet unknown ; but that its uses are 

 great, may be interred from this fact, that it 

 always exists in both kingdoms, and often in con- 

 siderable i)roportion. 



4. I had long ago ascertained that many strata 

 of limestone in Scotland, but especially those alloy- 

 ed with clay, contained various proportions of 

 magnesia. In these cases I always found the mag- 

 nesia to exist eitiier in tlie state of carbonate, or 

 combined with sulphur; and not unfrequently ex- 

 isting in both these states in the same rock. When 

 combined with sulphur, it effloresces into sulphate 

 of magnesia, which often forms a hoary incrusta- 

 tion on the lace of the rock. Yet the farmers, who 

 used the lime of these rocks in great quantities, 

 never entertained the smallest apprehension that it 

 was injurious to vegetation. 



5. 1 have analyzed various coal ashes, and never 

 failed to find in them sulphate of magnesia, and 

 sometimes also subcarbonate of magnesia, both, 

 not imfreouentlv, in considerable pronortions. Yet 



coal ashes arc applied to certain soils with very 

 good effect. 



6. Lord Dundonald having stated, in his essay 

 on Manures, that peat-ashes contained pot-ash, to 

 which he ascribed their virtues as manure, I was 

 led to analyze peat-ashes in various places from 

 Ayrshire to Caitliness, in the hope that those who 

 used peat-fuel might be able to furnish pot-ash 

 like those who burn wood. I never was able to 

 detect a single particle of pot-ash in the ashes of 

 peats; but I never failed to find sulphate of mag- 

 nesia, and frequently the sulphate and subcarbo- 

 nate in large proportions. Those peats which yield 

 white ashics contain most of tliese ingredients. 

 Now, peat-ashes are universally used as manure, 

 and mosses are often pared and burnt for the ex- 

 press purpose of manuring them with their own 

 ashes. An efflorescence of sulphate of magnesia 

 is often seen on moss binks, that is, the face of moss 

 from which people had been cutting peats. 



7. In the essay on manures, referred to in my for- 

 mer communication, I described a species of schistus 

 commonly known under the name of blaes, which 

 effloresces into sulphate of magnesia by exposure 

 to the air, and yields a considerable portion of sul- 

 phate and carbonate of magnesia from its ashes. I 

 had observed three species of schist i, one contain- 

 ing sulphur combined with clay, which oxygenates 

 into alum ; another containing sulphur united to 

 iron, which oxygenates into sulphate of iron ; and 

 the third contains sulphur united to magnesia, 

 which oxygenates into sulphate of magnesia. Often 

 all these three species are blended in the same mass; 

 and I have had evidence of the hurtful effects of 

 the aluminous and pyritical schistus, if laid upon 

 land before the salts are extracted. 



The magnesian schistus is commonly more hard 

 and compact than the others, is of a dark blue or 

 black color, and very much resembles parrot or 

 candle coal. I had once an opportunity of inspect- 

 ing an experiment on a large scale with this 

 schistus. Eeing set on fire by brushwood in very 

 large heaps, it burnt with a smouldering flame, 

 and left a great quantity of white ashes, such as 

 the country people call guests. These were found 

 to contain a very considerable proportion both of 

 the sulphate and subcaibonate of magnesia. The 

 ashes were laid upon the surface of land in pasture, 

 and allowed to remain until the lumps mouldered 

 down. When the land was ploughed, it yielded 

 very extraordinary crops, and the effects seemed 

 every way superior to tiiose of a good liming. 



I showed specimens of tiie sulphate of magnesia 

 in fine crystals, and also of pure magnesia, to the 

 late Dr. Black, which were extracted both from 

 the ashes, and from tiie efflorescence of this schistus, 

 together Vvitli specimens of the materials from 

 which they had been extracted ; and that illustri- 

 ous fiither of chemical science expressed an opinion 

 that this schistus might be advantageously used for 

 manufacturing the magnesia alba of the shops. 



My ingenious friend, Dr. Coventry, lately exhi- 

 bited specimens of this schistus in his class, and 

 mentioned that it was successfully used as a ma- 

 nure in some parts of Fii'eshire. 



I am aware that burnt clay operates on land as 

 an alterative, and renders it more penetrable both 

 by water and the roots of plants; and that the 

 whole effect is not to be ascribed to the magnesia, 

 or its salt. Lime operates both as an alterative, 

 and as a direct manure. Being capable of combin- 



