AGRICULTURAL CHEMISTRY.

Otago Witness, Issue 2106, 5 July 1894, Page 11

AGRICULTURAL CHEMISTRY.

A SBRIBS OP LECTURES DELIVERED

BY PROFESSOR BLACK AT THE grAtfu umvßßSitv, Vllli— Lxjib (Continued.)

U la necessary to distinguish between five tJnbsianees all containing lime (CaO) in different states of combination—

1. Carbonate of lime, as it occurs in calcspal 1 , Marble, Col'al, limestone, marl, stalactite*, stalagmit s, &c. In all these the lime is present in combination with carbonic acid gas (CO a ) and hence the name carbonate of lime.

2. Quick lime, or burnt lime, or simply Hm'. In this state the lime is got by the process known as the burning of limestone in limekiln?. From a chemical point of view it is absurd to talk of burning limestone. It is well enough known that limestone will not burn ; besides, in all cases of burning a uiroo is formed between the thing that is being burned and the oxygen of the air ; but iv what is called the burning of limestone there is nothing burned but the fuel that heats the kiln and its contents ; and, instead of the limestone forming a union with oxygt-n or anything else, it is a separation that takei place, as the 1 mestonemthe process parts with its carbonic acid gas, which escapes away into the air, leaving the lime itself behind in the form of quick lime. Ihe equation is

CaCO 3 = CaO + CO, ; which means that 100 tons of carbonate of lime when ha ted red hot for several days g ye off 44 tons of carbonic acid gas and leave a rtsidue of 56 tons of quick lime. 3 Slaked lime, or slacked lime, is quick lime combined with water, which has been poured on it in the process of slaking. Quick lime has a strong ttraction for water. In uniting with it the water is rapidly absorbed (appearing to an observer to be drying up), great heat is de-

main as hard cores of raw stone, (c) Again, to > high a temperature in the kiln at an early stage of the burning will, if there is sand or clay in the stone, be apt to fuse or "run" the surface, and especially the sharp edges of the lumps of stone, into the glassy slag mentioned above, thus obstructing the free escape of the gas. The temperature should therefore be raised gradually to the full red pitch, and there maintained f r a period depending on the size of tho pieces of stone, the Quantity of io in the kiln, and on its composition) If tho stone be nearly pure carbonate (like Oamaru, or the best "Waihola limestone), there is not much fear of overheating, ai pure lime itself cannot be fused by the heat of the kiln, and there islittlo else in the best qualities of these limestones to "l'un" with the lime Quicklime is tested to detect the presence of " un> burnt" c > rbonate as follows: — A pinch of the po a der — as much as will lie on a threepenny b t — taken from the very centre of the largest pieces^ is put into a wineglass ; three times its bulk of water is added, and then muriatic acid (spirits of salt) till the powder is all dissolved ; should there be any effervescence or bubbling or boiling up during the addition of the acid, it is a si n ttiat raw stone is present, and the briskness of the effervescence will be an indication of the quantity of such unburnt stuff.

A question might arise whether a given material is raw limestone or quick lime. This would be tested as follows : — A portion of the questionable part of ihe stone is crushed to small pieces ; these are shaken up in a bottle three-f urthjsfull of water, allowed to stand for a day, and then shaken up again and filtered through blotting-paper, or filter paper, in a funnel ; or, instead of filtering, the liquid is poured off clear after the sediment has all settled. If it was "unburnt," or raw, stone, the solu.io i will not have any particular taste, as io will be just ordinary water ; if, on the other hand, any part of it was iv the state of burnt lime, or quick lime, or slacked lime it will have a faiut caustic taste very perceptible, and if the breath be blown through it for two minutes (by inhaling through the nose and exhaling through a piece of hollow straw oj; other tube- let down three or four

dissolve. The chemicil symbols s>i these five kinds of "lime" are as follows :—: — I ' 1. Carbona'e of lime — CaC0 8 . 2. Quick lime— CLO. 3. Slaked lime— CaOH,O. 4. Limewater-CaOH-O+(H.O)x. i 5. Milk of lime-CaOH a O+(H,O)y. (In I Nos. 4 and 5 the value of y is less than that ofx.) The action of quick Hmo or Blamed lime in the soil may be described uuder c s heads us ! follows i— I 1, It supplies lime to plant) directly, as a necessary {.art of their mineral food. 2 Quick lime decomposes and destroys un° i desirable accumulations of organic matters, j such as tho brown masses of vege able debris ! (composed of roots, stems, leaves, &c , the remains of hundreds of generations of marsh 1 plants) that prove so great an obstacle in the reclamation of swamps. Before applying the lime for thisxurpose the swamp should first be ttoroughly dried by draining, and then the lime snould be liberally applied and at once mixed with the deliris in the " quick " or newlyburnt state It will now slake itself by taking water from the moisture of the organic matter and, being a strong caustic alkali, wH act chemically on the vegetable matter, disorganising it, comb'ning with part of nt, burning up another part, and laying it o^en to the oxidising action of those microbes whoselife work it is in the presence of lime or other alkali to consume organic matter, converting its carbon into carbonic acid and its nitrogen into nitric acid, both of which then combine with the lime, and thus furnish a very valuable contribution to the fertility of soil. Of coursa by burning the dried vegetable masses of swamp growth a similar resu t is more speedily obtained, as the ashes will contain carbonate of potash, which serves in this case as a good basis for microbe action, just as the lime did in the other case ; but here the nitrogen, which is doubtless the most valuable constituent of dead organic matter, is lost by the burning. This burnng action of quick lime on organic matter is utilised for the destruction of weeds in compost heaps. The weeds and other garden or field deh'is are collected and built up in alternate