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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Fkb. 10, 



Originally llic surface of the eartli coiisislod entirely of Iiaril rocks, wliich 

 by tile influence of moisture and oilier agents, liave gradually Ijecome disin- 

 tegrated and fitted lor the growth of plants. It is obvious, therefore, that 

 the nature of the soil must greatly depend upon the character of the rock 

 upon which the soil rests — though not always so, in consequence of ihe soil 

 from one rock being sometimes carried to a rock of a ditferent description. 

 In examining a geological map we find immense tracts of chalk, red sand- 

 stone, limestone, clay, coal, and so on. all of which give rise to diiTerent 

 soils ; and it is important to consider this branch of the sul.ject, for it ex- 

 plains to us how one system of amelioration which answers in one place, does 

 not succeed in another. 



The different strata give rise to what are usually called the different rocks, 

 the term " rock" being usually applied in agriculture to the base on which 

 the sub-soil immediately ties. The sub-.soil is the matter supposed to be de- 

 rived directly from the disintegration of the rock itself: then upon the top 

 of the sub-soil comes the superficial soil, which in its mineral contents will 

 agree with the rock and the subsoil, but which is nevertheless greatly dif- 

 ferent, in consequence of the animal matter which falls upon it, and the long 

 e.vposure to the air, mo sturo, heat and light. 



" 1 propose," says Mr. Brando, '• 1st. to lay'before you a short account of 

 the inorganic constituents of the soil, and endeavour to show how the pre- 

 ponderance of one or other of these constituents gives a different character to 

 the soil— in other words, what is meant by a sandy soil, a clayey soil, a 

 marly soil, a chalky soil, he. And in reference to these matters. 1 shall en- 

 deavour to limit myself to such an account as presses immediately upon 

 agriculture." 



There are four substances usually called earthy bodies which are met with 

 more or less in all fertile soils, and it is highly essential that an agriculturist 

 should be acquainted with their particular characters and peculiarities. They 

 are silica, or siliceous earth; alumina, or aluminous earth ; lime, or calca- 

 reous earth ; and magnesia; all of which arc resolved by the chemist in their 

 purest state to the form of a white powder. Chemically speaking, these sub- 

 stances are all metallic oxydes, and not as was formerly supposed, simple 

 boilics. With regard to sil ca. that very important ami abundant ingredient 

 in almost every soil, it has this curious constitution, that it is composed of 

 equal weights of a metallic body and oxygen. 



He would have his hearers particularly to bear in mind the following pro- 

 portions of the four earthy bodies which he had just referred to— all of them 

 consisting of a metal and oxygen, viz. : — 



8 parts of oxygon combine with 8 parts of the metal silicium, to form 



16 parts of oxyde of silicium, or silica. 

 8 parts of o.xygen combine with 9 of the metal aluminum, to form 17 parts 



of oxyde of aluminum, or alumina. 

 8 iwrts of oxygen combine with 20 of the metal calcium, to form 28 parts 



of lime. 

 And 8 parts of oxygen combine with 12 of the metal magnesium, to form 



20 parts of magnesia. 

 The Professor then observed that he would pass througli as quickly as he 

 could an outline of the properties of thes? bodies. To commence with s lica. 

 Silica exists in nature in a great variety of forms, absolutely, or very nearly 

 pure. We find it in rock crystal quite pure , very nearly so in flint, which 

 contains in addition some slight colouring matter, which we do not perfectly 

 understand, and perhaps about I per cent, of foreign matter. If you heat 

 flint, or rock crystal to a red heat, and then plunge it in water, it immedi- 

 ately becomes opaque, and can then be readily nibbed down to a fine powder. 

 Another very abundant source of silica is to be found in the white pebbles 

 H hich are so often met with in the beds of streams. And we also find a con- 

 siderable quantity of silica in the form of sand, of which we cannot take a 

 iietter specimen than the white sand from the western extremity of the Isle 

 of Wight, or from Lynn, in Norfolk. It is a curious fact with regard to 

 sand, if we examine it microscopically, we find it to consist partly of minute 

 crystals, and partly of small rounded particles, so that a part may be regarded 

 as the crystals of flints, and part as small pebbles. It would appear that 

 crystallized sand arises from decomposed granite — granite consisting of three 

 substances, quartz which is afterwards sand, felspar, and mica. 



The extraordinary property which strikes us as being very important with 

 regard to silica, is its utter insolubility in water and almost everything else. 

 You may keep even the finest sand you can obtain in water for any length 

 of time, and yet not the smallest porti.n will be dissolved. Silica, however, 

 does find its way into plants, and into some in considerable quantities— in 

 common straw, for instance, corn, and any grass, we find a great quantity of 

 this insoluble substance ; and it becomes a curious question to ascertain hoiv 

 silica is rendered soluble and finds its way into plants. .Silica is indeed of 

 the utmost importance to the texture of plants— if we take a stalk of wheat 

 we shall find that it is silica which gives it firmness, enables it to bear the car 

 in due season, ami imparts to it all those properties which belong to the 

 more perfect and better kinds of straw. But though silica is insoluble in 

 water, it is readily soluble in potass, soda, lime, and other alkalis. In some 



strata we observe silica presented to the roots of plants in a soluble form; 

 but if we dissolve silica in an alkali, and then throw it down again, we find 

 that in some cases it may be thrown down perfectly insoluble and sometimes 

 quite soluble. Sometimes it is taken up in a soluble form and sometimes re- 

 tained in an insoluble form. No doubt the silica in this wisp of straw Aas 

 hfen soluble, now it is perfectly insoluble in water. Glass is a compound of 

 silica and soda, yet glass is insoluble in water, for we know that it is employed 

 as a vessel for holding water. This arises from the mechanical texture of the 

 glass ; for if we rub glass to a fine powder, we find that it does become soluble 

 in w-ater. This application of silica to the soil becomes a very important 

 question ; because there is no doubt that certain crops fail, not for the want 

 of silica, because tliere shall be plenty, but for the want of it in a soluble 

 state. Now some agriculturists actually mix pounded glass, or another sub- 

 stance, which I will show you presently, with their manure, and employ it 

 with great success. If, instead of common glass, they take another com- 

 pjund of silica, viz., glass with more soda, they have a substance which 

 readily dissolves in water. The Lecturer here exhibited a solution — called by 

 the ancients "liquor of flints," and then proceeded to observe, "I can at 

 pleasure separate the silica from it in a particularly soluble, or in a particu- 

 larly insoluble slate," He then proceeded with the following experiment. In 

 a glass containing the " liquor of Hints " he added a large quantity of wafer : 

 and in another glass he had some of the liquor in a more concentrated state. 

 To each of these solutions, he added a little of almost any acid ; in the strong 

 solution silica was thrown down in the form of a jelly, and by adding more 

 water he showed that it would not re-dissolve the silica. In the weaker so- 

 lution no such result was obtained, although he added as much acid as to 

 the other, yet it remained perfectly clear. '• In the first state," said Mr. 

 Brande, " I need hardly tell you sili.a would be perfectly inert m the growth 

 of plants— in the other state it would be readily taken up. We find also, 

 though common glass in its usual state does not appear to be acted upon by 

 the air and water, that it does yield, when acted upon by those agents for a 

 long time. Pieces of glass are often met w ith in a field of all kinds of colours, 

 and so soft, that they will give way to the nail. 



There is another curious agent, which has the power of acting upon silica, 

 and of carrying it at once away, and wlien he comes to analyze soils, he 

 must show this substance — it is fluoric acid. Now it would appear that we 

 have to consider silica first as a mere mechanical ingredient of the soil— that 

 is, giving to the soil a certain looseness of texture possessed by all sandy 

 soils: and then, also, with reference to the component parts of the crops 

 growing upon the soil. 



Anotiier substance to which lie adverted, is the argillaceous earth or alu- 

 mina. It is an ingredient in all fertile soils, and from it they derive some 

 very important properties. In the first place, all clays contain alumina : and 

 he need not advert to the importance of clay in soils, and to the functions 

 which alumina performs in them. It is a very hydrometic substance, that is, 

 it has a great attraction for water, which it absorbs and retains in great 

 quantities. It is the only substance which gives plasticity when mixed with 

 other bodies; and whenever we have a plastic substance, then we have 

 alumina. 



Alumina is a most useful ingredient in the soil, provided that it does not 

 exist in excessive quantities; if there be too much of it, it forms that stiff, 

 clayey, unmanageable soil, of which there is so much in England. 1 must 

 take you to it in its pure state, and we get it out of alum. If we dissolve 

 a quantity of alum in water and add an alkali, we shall throw down alumina 

 in a pure state, or very nearly so. 



If we only take 10 or 12 per cent, of alumina, and mix it with silica, we 

 ihall find that it will give to it the plastic nature of clay. Indee I, a very- 

 small portion of alumina gives plasticity and adhesiveness and other important 

 properties, especially as relates to moisture, in any soil. Like silica, it also 

 forms a component part of the growing crop. Some vegetables cannot grow 

 without alumina ; none, perhaps, can grow well without it in the soil : some 

 few actually require it as fi od. There are certain vines which cannot be cul- 

 tivated without alumina ; in those vines we find alumina composing a certain 

 part of the plant — nay. we find it even in the grape— and even in the wine. 

 The Rhenish wines contain a considerable quantity of alumina ; and it is a 

 curious fact, that a quantity of wine was actually stopped at the Custom House 

 some time ago, which it was thou^^ht had been adulterated with alum, when, 

 in fact, it contained no more alumina than it had taken up from the soil. 

 Alumina at once difiers £rom silica, in that it is equally soluble in acid and 

 alkali. Another character of alumina is. that it combines with sulphuric acid 

 and potass, and crystallizes very readily ; but we have to deal with alumina 

 only in reference to the soil and its agricultural purposes. 



