tain elastic vapours and fluids produced, of a deleterious quality, and peculiar smell. The matter here alluded to is very visible to the naked eye in a sunbeam let into a dark room.

Why do flint and steel when struck together produce a shower of sparks?

Because small portions of one or both are struck off by the violence of the collision, in a state of white heat, and the particles of the iron burn in passing through the air: in a vacuum the heated particles are equally produced, but are scarcely visible from this combustion not occurring. In both cases they suffice to inflame gunpowder, or to light tinder.

EFFECTS OF HEAT.

Why do we stick a pin in a rushlight to extinguish it? Because the pin conducts away so much heat that the tallow will not melt, or rise in the wick.

Why does the heater of a tea-urn soon change when placed near the water?

Because it parts with its heat to the water, until both are of the same temperature.

Why are meat screens lined with tin?

Because the polished metal reflects the heat upon the roasting meat, and thus expedites the cooking, independently of the screen itself protecting the joint from currents of air. On this account, screens, entirely of tin, are calculated for expeditious cookery.

Why will that part of the curtains of a room which has been exposed to the sun, be often faded, while those parts which have not been so exposed retain their original colours?

Because the oxygen which existed in a solid form in the dye of the curtains, will be rendered aëriform by the rays of the sun, and will go off in the state of oxygen gas.

Why is a harp or piano-forte, which is well tuned in

a morning drawing-room, not perfectly in tune when a crowded evening party has heated the room?

Because the expansion of the strings is greater than that of the wooden frame-work; and in cold the reverse will happen. Arnott.

Why are urns for hot water, tea-pots, coffee-pots, &c, made with wooden or ivory handles?

Because, if metal were used, it would conduct the heat so readily that the hand could not bear to touch them; whereas wood and ivory are non-conductors of heat.

Why does a gate in an iron railing shut loosely and easily in a cold day, and stick in a warm one?

Because, in the latter, there is a greater expansion of the gate and railing than of the earth on which they are placed.

Why are thin glass tumblers less liable to be broken by boiling water, than thick ones?

Because the heat pervades the thin vessels almost instantly, and with impunity, whereas thicker ones do not allow a ready passage of heat.

Why will a vessel which has been filled to the lip with warm liquid, not be full when the liquid has cooled?

Because of the expansion of the fluid by heat. Hence some cunning dealers in liquids make their purchases in very cold weather, and their sales in warm weather.

Why is a glass stopper, sticking fast in the neck of a bottle, often released by surrounding the neck with a cloth taken out of hot water, or by immersing the bottle to the neck?

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Because the binding ring is thus heated and expanded sooner than the stopper, and so becomes slack or loose upon it.

Why does straw or flannel prevent the freezing of water in pipes during winter?

Because it is a slow conducting screen or covering,

and thus prevents heat passing out of the pipe. By the same means the heat is retained in steam pipes. Why have ice-houses double walls, and why do winecoolers consist of double vessels?

Because air fills the intervals between the walls or vessels; or in some cases the space is filled with straw, sawdust or charcoal, all which are nonconductors of heat.

Why have some houses double windows?

Because the air inclosed between the two windows greatly prevents the escape of heat which is produced within the house in winter. Thus, air is an imperfect conductor of heat. Houses which have double windows are likewise more quiet than others, from the air being also a bad conductor of sound.

EVAPORATION.

Why is a decanter of cold water when brought into a warm room, speedily covered with dew?

Because the temperature of the decanter is lower than that of the air immediately around it. The dew may be wiped off again and again, but will be constantly reproduced till the temperatures are equal. Upon this principle, the most convenient sort of hygrometer, or instrument for measuring the quantity of vapour in the atmosphere, is constructed.

Why are porous vessels used for wine-coolers?

Because, being dipped in water, they imbibe a quantity of it, which gradually evaporates; and, as a part of the heat necessary to convert the water into vapour will be taken from a bottle of wine placed in it, the wine is considerably cooled.

Why does the breath or perspiration of animals (of horses in particular, after strong exertion,) become strikingly visible in cold or damp weather?

Because the vapour (invisible while at a higher temperature) is thickly precipitated, by the air with

which it is mixed being too cold to preserve it invisible.

Why is profuse perspiration so cooling to labouring men, and all evaporation productive of cold?

Because of the necessity of a large quantity of caloric being combined with fluids, to convert them into vapour or gas.

Why do persons take cold by sitting in wet clothes? Because they suddenly lose a large portion of heat, which is carried off from the body by the evaporation of the water from the clothes.

Why, in hot countries, do persons continually throw water on curtains which there form the sides of apartments?

Because the evaporation of the water absorbs a vast deal of heat, and makes the apartments cool and refreshing.

Why are assembly-rooms ventilated?

Because of the motion produced by the changed weight of air, when heated. The air which is within the room becomes warmer than the external air, and the latter then presses it at every opening or crevice to displace the former,

Why does the sulphuric acid in fire bottles so often fail in igniting the matches?

Because the acid is continually attracting moisture from the air, owing to the imperfect manner of closing the bottles.

WATER.

Why do fluids always accommodate themselves to the shape of the vessels which contain them?

Because fluids want cohesion among their parts, whence they are incapable of assuming any particular form without external support.

Why is water never found of the highest purity? Because all natural waters are constantly coming into contact with some substance which they either

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dissolve or hold in suspension, arising from the great range of the affinity of water, and its peculiar action as a chemical agent.

Why is lime most generally contained in natural waters?

Because there are few springs which, during some part of their subterranean course, do not come in contact with calcareous earth, and there is no substance which appears so readily soluble in a variety of menstrua. The presence of lime uncombined in any natural waters is, however, conjectural.

Why is rain-water generally impure when collected in large towns?

Because it acquires a small quantity of sulphate of lime, and carbonate of lime, from the mortar of the roof, and plaster of the houses.

Why are ice and snow waters of superior purity? Because they contain no gas or air, or saline substances, such having been expelled during freezing. Why is the taste of common water pleasing and refreshing?

Because of the gases (carbonic acid and common air) which it contains.

Why does the best water exhibit the greatest number of air bubbles, when poured into a glass?

Because it contains the greatest quantity of carbonic acid gas, in addition to its proportion of atmospheric air.

Why do air bubbles in a glass of water dilate as they rise from the bottom to the surface?

Because the pressure of the liquor becomes less and less upon them.

Why is some water hard?

Because it contains calcareous salts, with carbonate and sulphate of lime; one grain of the latter, contained in 2,000 grains of soft water, being sufficient to convert it into the hardest water that is commonly met with. - Brande.