1844.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



73 



AGRICULTURAL CHEMISTRY. 



By Professor Brande, F.R S., &c. 



Lecture II.— February 3, 184i. 

 (Specially reported for this Journal.) 



The next most important constituent of the soil is lime. Now lime is found 

 in nature in various states of combination, but principally either as carbonate, 

 ph(is(ihate, or sulphate. As carbonate it exists in most fertile soils. It is 

 added, also, largely to fields as a fertilizing agent. But for this purpose it is 

 not used in combination, but in a free state, as caustic or quick lime, in vthich 

 condition it is obtnined by exposing limestone rocks, which are carbonate of 

 lime, to a strong beat in properly constructed kilns ; this drives off its car- 

 bonic acid, and reduces it to the state of pure lime. If chalk, for instance, is 

 heated, water first escapes, then carbonic acid, and the heat should be con- 

 tinued till it ceases to lose weight. As the limestones are not all pure, but 

 are mixed with other ingredients, so the resulting Hme is of diHerent qualities, 

 and receives various names, such as hydraulic lime, fat lime, and meagre 

 lime, each being a.lapled for some particular object, some being used largely 

 in mortars and cements. In lime-burning there are several circumstances 

 that must be attended to, or success will not be obtained; one is, that the 

 proper degree of heat be maintained, another, that a gooj current of air be 

 kept up. It has been found, by tlie experiments of Sir James Hall and others 

 that when carbonates of lime are heated to the most intense heat in closed 

 vessels, only a part of the carbonic acid escapes ; indeed, they have shown, 

 that II healed under pressure, none of the gas escapes from it, but that it is 

 converted into a semi-crystalline mass resembling marble ; in fact, it is sup- 

 posed that in this manner marble has been formed in nature, by the intense 

 heat of streams of volcanic lava acting on the chalk whilst imbedded beneath 

 other strata, the pressure not allowing the gas to escape, and during its slow 

 cooling assuming a crystalline arrangement. Hence we see the importance of 

 constructing the kiln so as to allow a good current of air to carry off the 

 carbonic acid as 'ast as liberated ; which is also assisted by the steam pro- 

 duced from the moisture in the chalk. Now the change of quality produced 

 by this burning „.;, by the old chemists, and is even by many of the un- 

 scientific agricu turists of the present day. attributed ,o something that it 

 abstracts from the hre, and hence, say they, its caustic quality. But Dr. 

 Black showed, ong ago, that such was not the case. He found that when 

 50 b of pure chalk were converted into lime, it only weighed 281b., hence 



nrnv;df r'"h 'm '"'i""' «"'• "'''^'' '^^'"^""^^^ experiments was 



Plowed o be carbonic acid, and the remaining 281b. of lime has been shown 



!^h' ,roT'r™""°^^°"'-°^=' metal which he named calcium, to- 

 gether wilh 81b. of oxygen gas, forming oxide of calcium, or lime. Hence 

 the composition of pure carbonate of lime may be represented thus :- 



,„ , (carbonic acid (P"'"'" •• G 



50 carbonate of lime-J ^^ oxygen 16 





lime 



/calcium . 20 

 ' (oxygon . . 8 



50 



When lime Is exposed to the air it crumbles down into a fine powder, and 

 If then examined it will be found to have combined with water from the air 

 and become what is termed slaked. The same effect may be produced by 

 pouring water on to lime. By taking a portion of fresh lime and pouring 

 water on it, it will be observed to swell considerably, and to become very hot 

 sufficiently so even to char wood and to fire gunpowder, at the same lime 

 Jailing into a dry powder, which weighs much more than i: did originally, 

 every 281b. of lime having combined with 91b. of water, forming 371b of 

 hydrate of lime or slaked lime. If left longer exposed, it is found then to 

 combine with the carbonic acid always present in the air, parting again with 

 the water, and returning to the state from which It set out, of carbonate of 

 lime. 



There are other methods of getting the carbonic acid from a carbonate 

 than by heat. If to 1,000 grains of chalk, an acid be added which has a 

 stronger attraction for the lime, the carbonic acid will be set free, and may 

 be collected as gas in a glass vessel inverted over water. This is the ordinary 

 method of obtaining carbonic acid for experiment. But the lime in this case 

 IS not obtained pure, as it combines with the acid employed. Then if an 

 equal weight of chalk be heate.i so as to drive off, in that manner, all its car- 

 bonic acid, and then placed in the glass vessel full of gas, it will be found that 

 alter a time the water will rise in the glass, showing that the lime is absorbing 

 the gas, and again becoming carbonate. 



Lime is soluble in water, and although it requires 800 times its weight of 

 water to dissolve it, the solution will exiiibit most of the properties ot lime. 

 It strongly reddens turmeric test paper, showing that it is alkaline : it absorbs 

 carbonic acid from the air, thn surface becoming covered with a crust of 

 chalk : aJded in excess to rain or river water, it produces a clou liness, prov- 

 ing that they contain-carbonic acid ; but if tlie water coiUaining the carbonic 

 acid 13 in excess, the carbonate first formed is redissolved, on account of the 



formation of the very soluble bi-carbonate of lime. To this latter property is 

 owing a beautiful appearance frequently met with in limestone districts ; the 

 rain, falling on the surface, becomes impregnated with carbonic aciil from 

 the soil, and then filtering through some crevice in the limestone, renders 

 some of it soluble ; it then, perhaps, finds an outlet at the roof of a cavern, 

 and here, being exposed to the air, parts with the extra quantity of carbonic 

 acid, depositing the insoluble carbonate of lime, which first encrusts the roof, 

 then by constant dripping, forms aseries of beautiful crystalline icicles termed 

 stactalites, the remainder falling on the floor, forms large stony masses termed 

 stalagmites. 



The consideration of these properties of quick lime will explain its utility 

 when added to the soil. Its first action when strewn on the field, is to absorb 

 moisture, to swell considerably, thereby loosening the texture of the soil- 

 then, when acted on by rain, to form a solution which is destructive to animal 

 life, killing all grubs and worms— when brouglit in contact with organic 

 matter, to decompose it, which maybe illustrated by mixing sawdust, lime 

 and water into a paste, when it becomes dark brown, evolving carbonic acid— 

 to decompose salts of alumina and iron which might be pernicious to the 

 young plant— and, ultimately, by uniting with carbonic acid, to diffuse car- 

 bonate of lime through ihe soil in a finer stale of division than it can be ob- 

 tained in by other processes. One of the uses of lime which has till lately 

 been overlooked, because taking place more slowly, is the property it has of 

 disintegrating several kinds of rock, such as felspar, clay slate, and mica 

 slate, setting free their alkali, which is highly necessary for vegetation. This 

 accounts for the power attributed to lime, by many agriculturists, of awaken- 

 ing the dormant energies of some soils, bringing them, in the course of four 

 or five years, into excellent condition. 



The tests for lime are few and simple. In limestones, the application of an 

 acid causes effervescence, as it generally exists in the state of carbonate. In 

 river water, such, for instance, as the Thames, it exists as carbonate, and on 

 boiling, is deposited, as is evident by the fur which accumulates in kettles. 

 In spring water, so that it is not too near the chalk, in our blue clay, for 

 instance, it is found as sulphate. Oxalate of ammonia is a very delicate test 

 for the presence of lime in solution, causing turbidness even when a very 

 minute portion is present. Ammonia causes no precipitate in solutions of 

 lime, and is therefore useful, in analysing soils, in'order to remove first those 

 substances which are precipitated by it, and then the addition of carbonate of 

 ammonia will throw down the lime. 



A fourth ingredient of the soil is magnesia, which though not in so great 

 quantity as the former, is still important This is found as carbonate in 

 some limestones, which are then termed dolomites, or miignesian limestones. 

 They are very excellent for building purposes, being very strong and durable, 

 and were highly recommended by the Commission appointed to select stones 

 for the Houses of Parliament. A curious point in their history relates to their 

 use as manure, in Yorkshire, where they abound. It was found that when 

 burnt for lime, they killed the young plant. Now this is owing to the fact 

 that the carbonate of magnesia, when burnt, is reduced to the caustic state, 

 or pure magnesia, similar to the lime ; also, where strewn on the soil, it ab- 

 sorbs carbonic acid, but so much more slowly than the lime, that when the 

 yonng plant shoots up, it still retains its causticity, and destroys it. This 

 difficulty is, by careful management, got over, and it is now much used. 

 Magnesia is found as pure carbonate in .some parts of Asia and America. It 

 is also a constituent of the serpentine rocks of Cornwall, and forms a large 

 part of steatite, augite. hornblende, and meerschaum. Many of these are 

 characterised by a peculiar greasy feel, hence steatite is well known by the 

 name of soap stone. It is also abundant in sea water; w/ien the salt has 

 been crystallized from it, it imparts a very bitter taste t • the residue, which, 

 on that account, is termed bittern. From this it is separated in l:irge quan- 

 tities and used in medicine as Epsom sails, or sulphite ot magnesia. Puro 

 magnesia is almost tast( less, but possesses a slight reddening power on lest 

 paper ; it is therefore an alkaline earih From solutions of magneslan salts, 

 carbonate of potash throws down the insoluble carbonate of magnesia, which 

 bears the same relation to pure magnesia that ch.ilk does to lime. Carbonate 

 of ammonia does not produce any precipitate, which may therefore be used 

 to separate lime from magnesia ; but if to the mixture phosphoric acid be added, 

 a precipitate is slowly deposited, which is therefore a very characteristic test 

 for magnesia. 



To these four earths, which are all metallic oxides, may be added, as a 

 common ingredient of the soil, oxide of iron. There are tivo oxides of iron, 

 the red, which is insoluble and consequently harmless, and the black, which is 

 very noxious. Both of these frequently impart colour to soils. The one is com- 

 monly known as rust of iron, the other, as slag. Dissolved inaci Is, and 

 lime or ammonia adiied, the respective oxides are precipitated combined with 

 water, as hydrates. But the lower or black oxiile has always a strong 

 tendency to pass into the higher state of oxidation, even by exposure to air. 

 This may olten be seen in ferruginous springs, which at their source are quite 

 clear, but as they flow a!ong, the protoxide of iron is converted into the 

 peroxiile, and is deposiied on ihe banks as an insoluble red powder. The best 

 tests whereby to recognise iron arc striking and delicate. One of the best is 



