FARMERS' REGISTER— MAGNESIA. 



429 



Headrick, however, may be disposed to say, that 

 this proportion is very unusual, as he seems never 

 to have met witli it. IMr. Tenant, in his memoir, 

 however, says, tliat he examined limestones from 

 six different parts of England, which contained 

 magnseia in this proportion, and mentions several 

 others, which, from the accounts he received, are 

 probably of tlie same nature. Hence it appears 

 likely, that this species of limestone is not of rare 

 occurrence, and of course to be found in Scotland, 

 although Mr. Headrick has not fallen in with it. 

 The contradiction of JMr. Headrick's friend's ex- 

 periment with the pure magnesia by Mr. Tenant, 

 only adds to the list of experiments carelessly 

 tried : He will probably be induced, from the above 

 extract, to doubt the accuracy of his result, and 

 take the troul)le of repeating it witli more care, 

 that he may either verify Mr. Tenant's results, or 

 detect his errors. 



Mr. Headrick's remark, that it must appear 

 strange to any one endowed with reason, that magne- 

 sia, which is evolved by the putrefaction oi'all animal 

 and vegetable substances, should operate as a poison 

 to plants, seems liable to a number of objections. I 

 shall only, however, give a general answer. A 

 certain proportion of magnesia may not be preju- 

 dicial, but even useful to plants, as we have reason 

 to suppose it is, from its always forming a compo- 

 nent part of them. But an overdose may here, 

 as in other cases of animals and vegetables, prove 

 mortal to vegetation. Water is a component part 

 of all vegetables, and is required in more or less 

 by every plant ; but the quantity useful to one 

 plant is destructive to another ; what would be 

 a moderate supply to a rush, would destroy heath. 



I shall here quote a remark of Dr. Pearson's, 

 contained in a communication made lo the Board 

 of Agriculture, on the use of green vitriol as a ma- 

 nure. Speaking of the qualities of the ashes con- 

 taining the green vitriol, he says, 'the quantity 

 spread on land is said to be fiity bushels per acre, 

 which I estimate at 22-50 pounds avoirdupois ; 

 this will give near seven ounces and a half per 

 square yard. If a larger quantity be applied, it is 

 observed it will prove extremely deleterious. This 

 is true also of every other manure, such as lime, 

 alkaline salts, marine salt, nay, of the dung of ani 

 mals ; lor if they be used in certain quantities, they 

 poison plants, instead of promoting their growth. 

 This is equally true in the animal kingdom, for 

 there is not an article taken as food or as seasoning, 

 which is not a poison, if taken in certain quanti- 

 ties. A human creature may be poisoned or ali- 

 mented by beef or pudding, according to the quan- 

 tity of them taken into the stomach. He may be 

 poisoned, or have digestion greatly assisted by salt 

 or pepper, according to their quantity. Jn brief, 

 the vulgar notion of the term poison is erroneous ; 

 for by it is conceived, that substances so called 

 are in their nature positively destructive of life; 

 but the truth is, that the most virulent poisons are, 

 in all reason and fact, only deleterious according to 

 the quantity applied. W hite arsenic, swallowed in 

 the quantity of len grains, or less, will destroy life; 

 biit m the quantity of one sixteenth of a grain, it 

 is as harmless as a glass of wine ; and further, in 

 that dose is a remedy for inveterate agues.' 



What has been asserted, therefore, concerning 

 the prejudicial qualities of magnesia, applies only 

 where the magnesia is in very considerable pro- 

 portion to the calcareous earth. In the course of 



my analysis of limestone, I have generally, in the 

 very best stratified specimens, defected a certain 

 proportion of this earth ; and from experience can 

 say, that where it exists in the proportion of one 

 tenth to the calcareous earth, such lime may be 

 used, upon fields of a light loam, at the rate of 

 500 Winchester bushels, slacked, per acre, with 

 great advantage. 



As to what is said by Mr. Headrick respecting 

 the use of ashes containing magnesia in the state 

 of sulphat, he must readily perceive that this by 

 no means determines against the prejudicial etlects 

 of carbonated or pure magnesia. Every one al- 

 lows the efficacy of pure or carbonated lime as a 

 manure; many doubt that of sul})hat of lime or 

 gypsum. Clay is a good soil ; but sulphat of clay, 

 or alum, (I take no notice of the portion of alkali 

 contained in this salt,) as Mr. Headrick himself 

 acknowledges, is destructive to plants ; wherefore, 

 although sulphat of magnesia may be useful to 

 vegetation, it does not follow, that pure or carbo- 

 nated magnesia is not hurtful. 



# * # » # 



A BAKFFSIIIRE AGRICULTURIST. 

 III. 



3Iagnesia not hurtful to Vegetation. 



I beg leave to express my obligations to your 

 Banilshire correspondent, for having brought for- 

 ward the original documents on which the alarm 

 respecting the deleterious effects of magnesia has 

 been founded. But though I give full and implicit 

 credit to the statements bothof 3/r. Smifhson Te- 

 nant, and of your correspondent, I am disposed 

 to draw conclusions from their premises very dif- 

 ferent from those which have occurred to them. 



It appears from Mr. Tenant's experiments, that 

 plants grew in the magnesia, and in the common 

 limestone, when reduced to powder, equally well as 

 they would have done in sand. In this case, I pre- 

 sume, both the lime and magnesia were in the state 

 of carbonate, or saturated with what was former- 

 ly called fixed air, now carbonic acid. It appears, 

 then, that the carbonate of magnesia, even when 

 combined with the carbonate of lime, exerted no 

 pernicious influence on the growth of plants. 



It was only when the carbonic acid was expell- 

 ed, by burning the stone, that the deleterious pro- 

 [)erties of the magnesia began to manifest them- 

 selves. Now, 1 do not see how pure magnesia can 

 exert any influence whatever on the growth of 

 plants. It is nearly insoluble in water, and hence 

 cannot enter into their roots. It recovers its car- 

 bonic acid much more slowly than lime ; and un- 

 less it meets with some vegetable, animal, or 

 mineral acid in the soil, with which it may com- 

 bhie, and become soluble, it is only after it has 

 combined with an excess of cai'bonic acid, that, 

 like lime, it becomes soluble in great quantity, 

 and can enter, with water, into the composition of 

 plants. 



But it is unnecessary to enter into any theoreti- 

 cal reasoning to account for the facts stated by Mr. 

 Tenant. It is well known that different soils re- 

 quire a very different management in the api)lica- 

 tion of lime. Some soils would be much bene- 

 fitted by small doses frequently repeated ; but 

 would be utterly destroyed by an excessive appli- 

 cation. On other soils, again, it is hardly possible 

 to lay too much lime. But I shall endeavor to il- 



