The ventilation of rooms and buildings can only be perfectly effected, by suffering the heated and foul air to pass off through apertures in the ceiling, while fresh air, of any desired temperature, is admitted from below. -Brande.

Why is the atmosphere of theatres, and other crowded places, prejudicial to health?

Because it has been found, that in a theatre, from the commencement to the end of the play, oxygen or vital air is diminished in the proportion of from 21 to 27, or nearly one-fourth, and is in the same proportion less fit for respiration than before.-Lavoisier.

Why are diving-bells restricted in their descent to ten or twelve fathoms?

Because of the density, and more especially the heat, of the air, although renewed by forcing pumps, becoming insupportable.

Why does the bark of trees resemble clothing?

Because it allows the heat to pass from the tree but slowly, and secures, therefore, the temperature necessary to vegetable life.

That caloric is as necessary for the support of vegetable as it is for that of animal life, may be proved by direct experiment. If, in the middle of winter, a hole be bored in a tree, and a thermometer put into it, it will be seen that the tree is many degrees warmer than the atmosphere.

Why does the effect of wind, or motion of the air, quicken evaporation?

Because it removes air saturated with the moisture, and substitutes air which is not, thus producing nearly the case of the substance placed in a vacuum.

Why do liquids evaporate neither so rapidly nor so greatly in air, as in a vacuum?

Because the presence of the air impedes the spreading from the liquid surface of the newly-formed vapour, and keeps it where its pressure resists the forma

tion of more vapour; and, because the air in contact with a liquid, shares its higher temperature with the liquid. Still, in India, flat dishes of water, placed through the night on beds of twigs and straw, kept wet, and in a current of air, soon exhibit thin cakes of ice and thus ice is procured in India, for purposes of luxury.-Arnott.

Why do heated sea-sand and soda form glass?

Because, by heating the mixture, the cohesion of the particles of each substance to those of its own kind is so diminished, that the mutual attractions of the two substances come into play, melt together, and unite chemically into the beautiful compound called glass. Why is sand used in glass?

Because it serves for stone; it being said, that all white transparent stones which will not burn to lime are fit to make glass.

Why do cracked glass vessels allow liquors to escape more or less?

Because of the various fissures, which are distinguished into four kinds. The first are such, that the liquid contained in the broken vessel escapes through them into the air. The second are such as retain the liquid, unless the vessel be immersed in water, or a similar fluid, and then the levels of the two portions of the fluid, tend to approach each other. The third are not permeable, (or, to be passed through) unless the fluids on opposite sides have a chemical action on each other: this is also the effect produced by the membrane of the bladder. The fourth kind are so fine, that no fluid passes, except in a single case.-Fischer. Why is fine table-glass very liable to wear and damage 2

Because of the great quantity of oxide of lead which it contains, to give it more density and refractive power. Its disadvantages are extreme softness, great fusibility, and liability to be corroded by acids. A

considerable quantity of this oxide is contained in that glass used for lustres, for artificial gems, and for most optical purposes. The superior appearance of Guinand's Swiss flint-glass is attributed to the quantity of oxide of lead which it contains.

ELECTRICITY.

Why is electricity so called?

Because of its origin from electron, the Greek name of amber, the phenomena displayed by rubbing a piece of amber, being the first physical fact recorded in the history of science.

Electricity investigates the attractions and repulsions, the emission of light and explosions, which are produced not only by the friction of vitreous, resinous, and metallic surfaces, but by the heating, cooling, evaporation, and mutual contact of a vast number of bodies.

Why are certain bodies called conductors of electrcity?

Because they suffer electricity to pass through their substance. The metals are all conductors; according to Mr. Harris, (Phil. Trans. 1827) silver and copper are the best conductors; then gold, zinc, and platinum, iron, tin, and lead. Well burned charcoal and plumbago also conduct.

Why are other bodies called non-conductors?

Because they only receive electricity upon the spo touched. Dry air, glass, sulphur, resins, and oils, are non-conductors. Water, damp wood, spirit of wine, damp air, some oils, and most animals and plants, are imperfect conductors.

Why have different bodies various conducting powers for electricity 2

Because their degree depends on the quantity of electricity which traverses them; so that of two conducting bodies, that which is the best for one electric current, may be the worst for either a stronger or a weaker current.-Professor Delarive, of Geneva.