Evening at Home 59

Evening at Home 59

Geo._ Yes, I have.

 

_Tut._ Were you not much surprised to see it swell and crack to pieces,

with a hissing noise and a great smoke and heat?

 

_Geo._ I was, indeed. But what is the cause of this—how can cold water

occasion so much heat?

 

_Tut._ I will tell you. The strong heat to which calcareous earth is

exposed in making it lime expels all the water it contained, (for all

earths, as well as almost everything else, naturally contain water,) and

also a quantity of a peculiar kind of air which was united with it. If

water be now added to this quicklime, it is drunk in again with such

rapidity, as to crack and break the lime to pieces. At the same time a

great heat is occasioned by the water combining with the lime, and this

makes itself sensible by its effects, burning all the things that it

touches, and turning part of the water to steam. This operation is

called slacking of lime. The water in which lime is slacked dissolves a

part of it, and acquires a very pungent harsh taste: this is used in

medicine under the name of lime-water. If instead of soaking quicklime

in water, it is exposed for sometime to the air, it attracts moisture

slowly, and by degrees fails to powder, without much heat or

disturbance. But whether lime be slacked in water or air, it does not at

first return to the state in which it was before, since it still remains

deprived of its air, and on that account is still pungent and caustic.

At length, however, it recovers this also from the atmosphere, and is

then mild calcareous earth as at first. Now it is upon some of these

circumstances that the utility of lime depends. In the first place, its

burning and corroding quality makes it useful to the tanner, in

loosening all the hair from the hides, and destroying the flesh and fat

that adhered to them. And so in various other trades it is used as a

great cleanser and purifier.

 

_Har._ I have a thought come into my head. When it is laid upon the

ground, I suppose its use must be to burn up the weeds?

 

_Tut._ True—that is part of its use.

 

_Geo._ But it must burn up the good grass and corn too?

 

_Tut._ Properly objected. But the case is, that the farmer does not sow

his seeds till the lime is rendered mild by exposure to the air and

weather, and is well mixed with the soil. And even then it is reckoned a

hot and forcing manure, chiefly fit for cold and wet lands. The

principal use of lime, however, is as an ingredient in _mortar_. This,

you know, is the cement by which bricks and stones are held together in

building. It is made of fresh slacked lime and a proportion of sand well

mixed together; and, when used for plastering walls, some chopped hair

is put into it. The lime binds with the other ingredients; and in length

of time, the mortar, if well made, becomes as hard, or harder, than

stone itself.

 

_Geo._ I have heard of the mortar in very old buildings being harder and

stronger than any made at present.

 

_Tut._ That is only on account of its age. Burning lime and making

mortar are as well understood now as ever: but in order to have it

excellent, the lime should be of a good quality, and thoroughly burnt.

Some sorts of lime have the property of making mortar which will harden

under water, whence it is much valued for bridges, locks, wharfs, and

the like.

 

_Geo._ Pray, is not plaster of Paris a kind of lime? I know it will

become hard by only mixing water with it, for I have used it to make

casts of.

 

_Tut._ The powder you call plaster of Paris is made of an earth named

_gypsum_, of which there are several kinds. _Alabaster_ is a stone of

this sort, and hard enough to be used like marble. The gypseous earths

are of the calcareous kind, but they have naturally a portion of acid

united with them, whence they will not effervesce on having acid poured

on them. But they are distinguished by the property, that after being

calcined or burnt in the fire, and reduced to powder, they will set into

a solid body by the addition of water alone. This makes them very useful

for ornamental plasters, that are to receive a form or impression, such

as the stucco for the ceiling of rooms.

 

Well—we have said enough about calcareous earths; now to another class,

the _argillaceous_.

 

_Geo._ I think I know what those are. _Argilla_ is _Latin_ for _clay_.

 

_Tut._ True; and they are also called _clayey_ earths. In general, these

earths are of a soft texture and a sort of greasy feel; but they are

peculiarly distinguished by the property of becoming sticky on being

tempered with water, so that they may be drawn out and worked into form

like a paste. Have you ever, when you were a little boy, made a

clay-house?

 

_Geo._ Yes, I have.

 

_Tut._ Then you well know the manner in which clay is tempered, and

worked for this purpose?

 

_Har._ Yes—and I remember helping to make little pots and mugs of clay.

 

_Tut._ Then you imitated the potter’s trade; for all utensils of

earthenware are made of clays either pure or mixed. This is one of the

oldest arts among mankind, and one of the most useful. They furnish

materials for building, too; for bricks and tiles are made of these

earths. But in order to be fit for these purposes, it is necessary that

clay should not only be soft and ductile while it is forming, but

capable of being hardened afterward; and this it is, by the assistance

of fire. Pottery-ware and bricks are burnt with a strong heat in kilns,

by which they acquire a hardness equal to that of the hardest stone.

 

_Geo._ I think I have heard of bricks being baked by the sun’s heat

alone in very hot countries.

 

_Tut._ True; and they may serve for building in climates where rain

scarcely ever falls; but heavy showers would wash them away. Fire seems

to change the nature of clays; for after they have undergone its

operation, they become incapable of returning of themselves to a soft

and ductile state. You might steep brick-dust or pounded pots in water

ever so long without making it hold together in the least.

 

_Geo._ I suppose there are many kinds of clays?

 

_Tut._ There are. Argillaceous earths differ greatly from each other in

colour, purity, and other qualities. Some are perfectly white, as that

of which tobacco-pipes are made. Others are blue, brown, yellow, and in

short of all hues, which they owe to mixtures of decaying vegetable

substances or metals. Those which burn red contain a portion of iron. No

clays are found perfectly pure; but they are mixed with more or less of

other earths. The common brick-clays contain a large proportion of sand,

which often makes them crumbly and perishable. In general, the finest

earthenware is made of the purest and whitest clays; but other matters

are mixed in order to harden and strengthen them. Thus _porcelain_ or

_china_ is made with a clayey earth mixed with a stone of vitrifiable

nature, that is, which may be melted into glass; and the fine pottery

called _queen’s ware_ is a mixture of tobacco-pipe clay, and flints

burnt and powdered. Common stone _ware_ is a coarse mixture of this

sort. Some species of pottery are made with mixtures of burnt and

unburnt clay; the former I told you before, being incapable of becoming

soft again with water like a natural clay.

 

_Har._ Are clays of no other use than to make pottery of?

 

_Tut._ Yes, the richest soils are those which have a proportion of clay;

and marl, which I have already mentioned as a manure, generally contains

a good deal of it. Then clay has the property of absorbing oil or

grease, whence some kinds of it are used like soap for cleaning clothes.

The substance called _fullers’ earth_ is a mixed earth of the

argillaceous kind; and its use in taking out the oil which naturally

adheres to wool is so great, that it has been one cause of the

superiority of our woollen cloths.

 

_Har._ Then I suppose it is found in England?

 

_Tut._ Yes. There are pits of the best kind of it near Woburn in

Bedfordshire, and Nutfield in Surrey, England. The different kinds of

slate, too, are stones of the argillaceous class; and very useful ones,

for covering houses, and other purposes.

 

_Har._ Are writing slates like the slates used for covering houses?

 

_Tut._ Yes; but their superior blackness and smoothness make them show

better the marks of the pencil.

 

_Geo._ You have mentioned something of sand and flints, but you have not

told us what sort of earths they are.

 

_Tut._ I reserved that till I spoke of the third great class of earths.

This is the _siliceous_ class, so named from _silex_, which is Latin for

a flint-stone. They have also been called _vitrifiable_ earths, because

they are the principal ingredient in glass, named in Latin _vitrum_.

 

_Geo._ I have heard of flint-glass.

 

_Tut._ Yes—but neither flint, nor any other of the kind, will make

glass, even by the strongest heat, without some addition; but this we

will speak of by-and-by. I shall now tell you the principal properties

of these earths. They are all very hard, and will strike fire with

steel, when in a mass large enough for the stroke. They mostly run into

particular shapes, with sharp angles and points, and have a certain

degree of transparency, which has made them also be called _crystalline_

earths. They do not in the least soften with water, like clays; nor are

they affected by acids, nor do they burn to lime, like the calcareous

earths. As to the different kinds of them, _flint_ has already been

mentioned. It is a very common production in some parts, and is

generally met with in pebbles, or round lumps forming pebbles, in

gravel-beds, and often almost entirely covering the surface of ploughed

fields.

 

_Har._ But do they not hinder the corn from growing?

 

_Tut._ The corn, to be sure, cannot take root upon them, but I believe

it has been found that the protection they afford to the young plants

which grow under them is more than equal to the harm they do by taking