lime. Direct experiments must decide further on this head. One thing more may be observed, that the supposed injurious property of burnt magnesia as regards the growth of plants, is imaginary, for since 10, 20 and more hundred weight have been put on 1 acre of land, and this for many years past, in the manuring with Saxon 'lowland' lime, without injury and indeed with advantage over pure lime, that opinion may certainly be regarded as erroneous.

Marl.

Marl is a lime-mud deposited in many low situations, in the last overflowings which have taken place on the surface of the globe, sometimes tolerably pure, sometimes more or less intermingled with clay, loam, stones, shells, &c. In particular, rare cases, however, it has been produced from limestones or rocks containing much lime, which the influence of weather has changed into an earthy mass, or at least so far decomposed, that they fall into this condition when laid open to the air, especially when frosts assist the action. In this substance lime is contained in the state of carbonate, the quantity of which varies extremely, from a few parts to 70 or 80 per cent. The kinds richest in lime are called lime or chalk marl, those composed of clay and lime, clay marl, those consisting of loam and lime, loam marl, and those of sand and lime, sandy marl. When quick lime is allowed to lie long exposed to the air, it gradually re-absorbs from the latter the carbonic acid which had been driven off by burning, and becomes again carbonate of lime, which however is distinguished from the original by its finely-divided, pulve rulent consistence. In this condition it may be regarded as an exceedingly rich lime-marl. The same thing takes place, and usually much more rapidly, in the soil, and hence after a longer or shorter period, after a manuring with lime, we must look for the latter in the soil in the condition of marl or carbonate of lime.

Numerous experiments and researches go to show that the most favourable composition, in reference to the amount of lime of an arable soil, is where it contains about 8 or 10 lbs. of carbonate of lime in 100 lbs. With this agrees very well the composition, only recently ascertained accurately, of the most fertile land in Saxony, namely the

loamy soil of the Lomatz district, in which the analyses of specimens taken at intervals along a distance of 8 leagues, showed an almost uniform proportion from 8 to 9 per cent. of carbonate of lime. Therefore if we bring large quantities of carbonate of lime or marl into a soil which is poor in this ingredient, we improve the composition of the soil, and this the more completely, when the other constituents of the marl contribute to compensate extreme conditions of soil; consequently when we lay clay- or loam-marl on light soils, and sandy marl on heavy ones. According to the practice of countries which make large use of marling, an addition of 100 to 150 tons per acre is regarded as but moderate. With such a quantity, even if the marl only contained 20 to 25 per cent. of lime, the amount of carbonate of lime in the soil would be raised at least 2 to 3 per cent., an increase which ́must have a favourable influence on the external or physical properties of the soil, that is, its solidity and porosity, and its behaviour in relation to air, water, heat, &c. Since the · pulverulent earthy lime, like humus, is capable of making heavy, tenacious soils more permeable and looser, and light soils more close and coherent, it follows that marling may prove useful in both cases.

Besides this effect, which I look upon as the principal, marl may, in particular cases, act also through its own peculiar chemical properties. It has been stated above, under quick lime, that the effect of the latter may be divided into two periods, namely, a, the first, when it acts as caustic lime, and b, the second, when, after it has absorbed carbonic acid from the air and earth, it acts as carbonate of lime. Carbonate of lime is an essential ingredient of marl; its further effects will therefore agree with the second or ulterior effects of quick lime. Among these are: that it has the power of neutralizing sour soils, and hence furnishes a simple means of so changing peat, moor-soil or mould, as to render it applicable as a manure and an ameliorator of the soil; that it furnishes plants with lime, in cases where the soil is deficient in this; and that enables the soil to convert the nitrogen of the decaying organic substances contained in it, into nitric acid.

Lastly, it cannot be doubted that the secondary constituents unfolded in many kinds of marl, must claim a share

in the efficiency which they have the power of displaying. Most kinds of marl, namely, contain variable quantities of organic substances, nitrogen, phosphoric acid, sulphuric acid and potash. Although these may at first sight seem too inconsiderable to claim much for them and their effects, they not unfrequently advance to figures quite deserving of notice, when we calculate them in the large masses of marl which are commonly used as manure. Thus, I found in Saxon marl:

If 50 tons of this marl are placed upon 1 acre of land, we give thus in the marling, in addition to the lime and magnesia, the following quantities of other manuring substances (in approximative numbers):

When these figures are compared with those stating the composition of stable-dung and animal excrements, it will no longer be found surprising that such a marling acts in the first year like manuring, with stable-dung. If then assisted with nitrogenous manures, for example, Chilian nitre, sulphate of ammonia or Peruvian guano, more rich crops may be here again obtained without any care for absorption; for the quantity of mineral substances given to the soil with 50 tons of such marl, still exceeds very considerably that which has been added to the soil in 50 tons of stablemanure. But without such contribution of nitrogenous manure, marl-just like lime-will make "poor children,"

as, in complete agreement with this, experience has already taught. If the marl contains no phosphoric acid and no potash, these must of course be returned to the soil the more abundantly in proportion to its poverty in this respect, and in proportion to the increased production forced on by the marling, or the fertility of the soil will be diminished. As a further confirmation of the asserted importance of the secondary constituents of marl, I may mention also the manuring with what is called clay-earth or diluvium, in marshy districts. In several specimens of such earths, which I recently obtained from Oldenburg, I found among other matters:In 1000 parts.

Lime and Magnesia. Phosphoric acid.

When we look at the gradation existing between these figures, the conjecture forces itself on us, that we must attribute a share of the effect of these two manuring earths to the phosphoric acid as well as to the lime.

Six years ago Saxony was still supposed to be destitute of marl; now more than a dozen, some very large and rich, beds are known, since they have been sought for. But this treasure is not yet dug out, at least by no means generally, for dear old custom always prefers to turn to the quick lime which has been in use from old times. Probably such treasure still lies hidden in the ground in many other places; it appears only to require the divining rod to indicate the places where it lies, and the touchstone by which it can be ascertained whether it really is, what it appears to be. both are quite close at hand: the divining rod is called "look for it," and a wine-glass of "hydrochloric acid" serves as touchstone. How many accidental opportunities the farmer has of penetrating a little deeper than usual into the earth! Here a well is dug, or a ditch; there a hill is levelled or cut through in road-making; in other spots a stone-quarry, a sand or loam pit opened. These are all excellent opportunities—and even deep-ploughing and ordinary work with

Yet

the subsoil-plough not unfrequently furnish others to make acquaintance with the kind of earth lying beneath the cultivated soil. If an earth of different character is met with under the surface-soil, a few drops of hydrochloric acid should be poured upon a specimen of it; if this produces a boiling and effervescence, it is a sure sign of the presence of lime, and the earth probably belongs to the useful kinds of marl, which may then readily be ascertained more exactly by a chemical examination.

XIII. GYPSUM. MANURING SALT. COMMON SALT.

GYPSUM AS A MANURE.

MANURING with gypsum has now been generally known for eighteen years, and we possess experience of it from all parts of the world; it might be thought from this that practice had arrived at a complete clearness as to its effect and application. Gypsum contains only two constituents, lime and sulphuric acid; and these do not require previous weathering, but only the presence of water, to render them soluble and assimilable by plants; it might be thought from this that theory had arrived at a clear and certain perception of the causes which lie at the base of the action of gypsum. Neither, however, of these things is the case; practice cannot state definitely and unanimously where and when it does or does not act, and theory cannot yet explain definitely and certainly how and why it does or does not produce effect.

There has been no deficiency of endeavours to make out the mode of action of gypsum; we are rather indebted to them for a superfluity of explanations, which, however, all agree in this, that they suit excellently to particular places, but are very forced as applied to others. Far too little has been done on the chemical side in the deeper study of the