1838] 



FARMERS' REGISTER 



653 



on the cliemistiy of iii^riciilttiro; yet as the theory 

 of the operation of Mme in this way is not Cully 

 elated in any elementary book that I have pe- 

 rused, I sliai! say a very few words on the appli- 

 cations ol' this part of chemical knowledge. 



There are two modes in which lime acts as a 

 cement; in its comhmalion with water, and in its 

 combinaMon with carbonic acid. 



The hydrate ol' lime has been already mention- 

 ed. Wlien quicklime is rapidly made into a paste 

 with water, it soon loses its softness, and the wa- 

 ter and the lime form together a solid coherent 

 mass, which consists, as has been stated bolbre, 

 of 17 parts of water to 55 parts of lime. When 

 hj'draie of lime whilst it is consolidating is mixed 

 ' with red oxide of iron, alumina, or silica, the mix- 

 ture becomes harder and more coherent than when 

 lime alone is used; and it appears that this is 

 owins to a certain degree of chemical attraction 

 between hydrate of lime and these bodies; and 

 they render it less liable to decompose by the ac- 

 tion of the carbonic acid in the air, and less solu- 

 ble in water. 



The basis of all cements that are used for works 

 which are to be covered with water must be ibrm- 

 ed from hydrate of lime; and the lime made li'om 

 impure limestones answers this purpose very well. 

 Puzzolana is composed principally of silica, alu- 

 mina, and oxide of iron; and it is used mixed with 

 Hrae, to form rements intended to be employed un- 

 der water. Mr. Smeaton, in the construction of 

 the Eddystone lighthouse, used a cement com- 

 posed of equal parts by weight of slacked lime 

 and puzzolana. Puzzolana is a decomposed lava. 

 Tarras, which was formerly imported in conside- 

 rable quantities from Holland, is a mere decom- 

 posed basalt: two parts of slacked lime and one 

 part of tarras form the principal part of the mor- 

 tar used in the great dikes of Holland. Sub- 

 stances which will answer all the ends of puzzo- 

 lana and tarras are abundant in the British is- 

 lands. An excellent red tarras may be procured 

 in any quantities from the Giants' Causeway in 

 the north of Ireland; and decomposing basalt is 

 abundant in many parts of Scotland, and in the 

 northern districts of England in which coal is 

 found. 



Parker's cement, and cements of the same kind 

 made at the alum works of Lord Dundas and 

 Lord Mulgrave, are mixtures of calcined ferrugi- 

 nous, siliceous, and aluminous matter, with hy- 

 drate of lime. 



The cements which act by combining with car- 

 bonic acid, or the common mortars, are made by 

 mixing together slacked lime and sand. These 

 mortars at first solidify as hydrates, and are slowly 

 converted into carbonate of lime by the action of 

 the carbonic acid of the air. Mr. Tennant found 

 that a mortar of this kind in three years and a 

 quarter had regained 63 per cent, of the quantity 

 of carbonic acid gas which constitutes the definite 

 proportion in carbonate of lime. The rubbish of 

 mortar from houses owes its power to benefit lands 

 principally to the carbonate of lime it contains, 

 and the sand in it; and its state of cohesion ren- 

 ders it particularly fitted to improve clayey soils. 



The hardness of the mortar in very old build- 

 ings depends upon the perfect conversion of all its 

 parts into carbonate of lime. The purest lime- 

 stones are the best adapted tor making this kind of 

 mortar; the magnesian limestones make excellent 



water cements, but act with too little energy 

 upon carbonic acid gas to make good common 

 mortar. 



The Romans, according to Pliny, made their 

 best mor'ar a year before it was unod; so that it 

 was partially combined with carbonic acid gas bc- 

 Ibre it was employed.* 



In biirning lime there are some particular pre- 

 cautions re(]uired (or the different kinds of lime- 

 stones. In general, one bushel o( coal is sulli('ient 

 to make f()ur or five bushels of lime. The mag- 

 nesian limestone requires less fuel than the com- 

 mon limestone. In all cases in which a limestone 

 containing much aluminous or siliceous earth is 

 burnt, great care should be taken to prevent the 

 fire from becoming too intense; for such lime easi- 

 ly vitrifies, in consequence o( the affinity of lime 

 for silica and alumina. And as in some places 

 there are no other limestones than such as contain 

 other earths, it is important to attend to this cir- 

 cumstance. A moderately good lime may bo 

 made at a low red heat; but it will melt into a 

 glass at a white heat. In lime-kilns for burning 

 such lime, there should be always a damper. 



In general, when limestones are not magne- 

 sian, their purity will be indicated by their loss of 

 weiirht in burning; the more they lose, the larger 

 is the quantity of calcareous matter they contain. 

 The magnesian limestones contain more carbonic 

 acid than the common limestones; and I have 

 found all of them lose more than half their weio-ht 

 by calcination. 



Besides being used in the forms of lime and 

 carbonate of lime, calcareous matter is applied for 

 the purposes of agriculture in other combinations. 

 One of these bodies is gypsum or sulphate of lime. 

 This substance consists of sulphuric acid (the 

 same body that exists combined with water in oil 

 of vitriol) and lime; and when dry it is composed 

 of 55 parts of lime and 75 parts of sulphuric acid. 

 Common gypsum or selenite, such as that found 

 at Sholover Hill near Oxford, contains, besides 

 sulphuric acid and lime, a considerable quantity of 

 water; and its composition may be thus express- 

 ed:— 



Sulphuric acid, one proportion - 75 



Lime, one proportion - - - 55 

 Water, two proportions - - 34 



The nature of gypsum is easily demonstrated: 

 if oil of vitriol be added to quicklime, there is a 

 violent heat produced; when the mixture is ig- 

 nited, water is given off, and gypsum alone is the 

 result, if the acid has been used in sufficient quan- 

 tity; and gypsum mixed with quicklime, if the 

 quantity has been deficient. Gypsum free from 

 water is sometimes found in nature, when it is 

 called anhydrous selenite. It is distinguished 

 from common gypsum by giving ofT no water 

 when heated. 



When gypsum, free from water, or deprived of 

 water by heat, is made into a paste with water, it 

 rapidly sets by combining with that fluid. Plas- 

 ter of Paris is powdered dry gypsum,t and its 

 property as a cement, and in its use in making 

 casts, depends upon its solidifying a certain quan^ 



* Pliny, speaking of flie qualities of mortars, says, 

 "Infrita quoque quo vetustior eo melior. In antiqua- 

 rum edium legibus invenitur, ne recentiore trima ute- 

 retiir redemptor." Nat. Hist. lib. xxxvi. cap. 23.— 

 J. D. 



t Anhydrous gypsum. 



