428 



FARMERS' REGISTER— MAGNESIA. 



ing with large doses of carbonic acid and oUier 

 compound gasses, it retains tliese in the soil, and 

 easily gives them over to the roots of plants ; and 

 being very soluble with an excess of carbonic acid, 

 it may easily enter the plants along with it. I am 

 inclined to think that magnesia always enters 

 plants in union with some acid, because, in its pure 

 state, it is insoluble in water. But, whatever be 

 the mode by which it enters, it certainly does enter 

 plants ; and hence it makes part of their food, or is 

 a maimre. In the case under consideration, if we 

 be not entitled to ascribe all the merit to the mag- 

 nesia, or its combination with sulphuric acid, it 

 must at least be admitted that the presence of these 

 ingredients does no harm. 



It is hardly necessary to mention, that the re- 

 fuse of salt is often used as a manure. It seems 

 now to be ascertained tliat marine salt contributes 

 nothing to the nourislmnent of plants, those only 

 excepted which grow in the sea. But the muriate 

 and sulphate of magnesia, and other earthy salts, 

 which abound in the refuse of sea-salt, certainly 

 promote the growth of plants, and are hence ap- 

 plied as manures. 



9. An ingenious friend of mine tried the pure 

 magnesia of the apothecaries, and found that it 

 neither did good nor harm to vegetables. 



From all these lacts I am warranted to conclude, 

 that magnesia, in certain combinations, so far from 

 being a poison, is a manure, or pabulum of plants; 

 that, when pure, it is inert, doing neither good nor 

 harm. If there be any combinations of magnesia 

 which are hurtful to vegetation, it becomes those 

 who have raised the cry against it to point out 

 what these combinations are, or hold their tongues. 

 It cannot be any state or combination of magnesia 

 that has ever occurred to me in analyzing lime- 

 stones, or other substances used as manures ; for, 

 in all these cases, the presence of magnesia, instead 

 of being hurtful, was apparently highly beneficial. 



I remain, sir, your most obedient servant, 



JAMES HEADRICK. 



Edinburgh, Nov. 2, 1804, 



II. 



Strictures on Mr. Headrick's Letter concerning 

 Magnesian Limestone. 



Mr. Headrick, in your 20th number, having 

 made some remarks on the prejudicial qualities of 

 magnesian limestone, wliich, in my judgment, 

 have a tendency to mislead many of your readers, 

 as coming from a j)erson who seems possessed oi 

 a considerable degree of chemical knowledge, per- 

 mit me to state a few things in reply. 



The original memoir concerning the prejudicial 

 qualities of pure or carbonated magnesia, when 

 combined with lime, and made use of in agricul- 

 ture, Mr. Headrick says lie has never seen. I 

 beg leave, therefore, to make some extracts from 

 this paper, which, I ai)prehend, may give Mr. 

 Headrick some account of the rise of the magne- 

 siaphobia, as he is pleased ludicrously to term it ; 

 and perhaps, considering the very high character 

 of the author, may convince him that is by no 

 means so groundless as he is disposed to imagine. 

 Full information of all that is known concerning 

 the bad effects of magnesia, may have a tendency 

 likewise to remove the improper dread of the com- 



binations of mac^iesia and lime, of which Mr. 

 Headrick complains. 



In the Transactions of the Royal Society of 

 London, 1799, Part II. p. 30.5, Mr. Srmthson Te- 

 nant, a gentleman well known as a most accom- 

 plished chemist, first called the attention of the 

 public to the magnesian limestone. He was in- 

 formed " that, in the neighborhootl of Doncaster, 

 two kinds of lime were employed in agriculture, 

 whicli were supposed to differ materially in their 

 effects. One of these, which was procured near 

 the town, it was necessary to use sparingly, and to 

 spread very evenly over the land ; for it was said 

 that a large proportion of it, instead of increasing, 

 diminished the fertility of the soil ; and that wher- 

 ever a heap of it was left in one spot, all vegetation 

 was prevented for many years. The other sort of 

 lime, which was obtained from a village near Fer- 

 rybridge, though considerably dearer, from the 

 distant carriage, was more frequently employed 

 on account of its superior utility. A large quanti- 

 ty v/as never found to be injurious ; and the s})ots 

 which were entirely covered with it, instead of 

 being rendered barren, became remarkably fer- 

 tile. The different properties ascribed to these 

 two kinds of lime were so very distinct, that it 

 seemed probable they could not be imaginary, and 

 it therefore appeared to be worth the trouble of as- 

 certaining them more fully, and of attempting to 

 discover the nature of the ingredients from whence 

 the difference arose. For this purj)Ose, I procured 

 some pieces of each sort of limestone, and first 

 tried what woidd be their effect upon vegetables 

 in their natural state, by reducing them to coarse 

 powder, and sowing in them the seeds of different 

 plants. In both kinds the seeds grew equally 

 well, and nearly in the same manner as they 

 would in sand, or any other substance which affords 

 no nourishment to vegetables. Pieces of each 

 sort of stone were then burnt to lime ; and after 

 they had been exposed some weeks to the air, that 

 their causticity might he diminished, .some seeds 

 were sown in them. * In the kind of lime which 

 was found most beneficial to land, almost all 

 the seeds came up, and continued to grow as 

 long as they were supplied with water ; and the 

 roots of the j)lants had many fibres w hich had pen^ 

 etrated to the bottom of the cup in which they 

 grew. Upon examining the composition of this 

 sort of lime, it proved to consist entirely of calca- 

 reous earth. By its exposure to the air for about 

 three months, it was found to have absorbed four 

 fifths of the fixed air required to saturate it. In 

 the other kind, a few only of the seeds grew ; and 

 the plants produced from them had hardly any 

 stalks or roots, being formed almost entirely of the 

 two seed-leaves, which lay quite loose upon the 

 surAice. This sort of lime being spread upon a 

 garden soil, to the thickness of about a tenth of an 

 inch, prevented nearly all the seeds which had 

 been sown from growing up, whilst no injury was 

 occasioned by common lime used in the same man- 

 ner. Upon examining the composition of the sub- 

 stance, which was so destructive to the plants, it 

 was discovered to contain three parts of pure cal- 

 careous earth, and two of magnesia." 



The above detail will, I think, be sufficient to 

 satisfy an)' one, that pure or carbonated magnesia, 

 when combined with lime, in the proportion of two 

 parts to three, is prejudicial to vegetation, when 

 used in a considerable quantity as a manure. Mr, 



