The ornamental alum baskets, whose manufacture was once so favourite a pursuit of lady-chemistry, were made upon this principle; the forms of the baskets being determined by wire framework, to which the crystals readily adhere.

Why is sugar-candy sometimes in large and regular crystals?

Because the concentrated syrup has been kept for several days and nights undisturbed, in a very high temperature; for, if perfect rest and a temperature of from 120 deg. to 190 deg. be not afforded, regular crystals of candy will not be obtained.

The manufacture of barley-sugar is a familiar example of crystallization.  The syrup is evaporated over a slow heat, till it has acquired the proper consistence, when it is poured on metal to cool, and when nearly so, cut into lengths with shears, then twisted, and again left to harden.

Heat.

Why does hay, if stacked when damp, take fire?

Because the moisture elevates the temperature sufficiently to produce putrefaction, and the ensuing chemical action causes sufficient heat to continue the process; the quantity of matter being also great, the heat is proportional.

Why is the air warm in misty or rainy weather?

Because of the liberation of the latent heat from the precipitated vapour.

Why is heated air thinner or lighter than cold air?

Because it is a property of heat to expand all bodies; or rather we should say, that we call air hot or cold, according as it naturally is more or less expanded.

Why is a tremulous motion observable over chimney-pots, and slated roofs which have been heated by the sun?

Because the warm air rises, and its refracting power being less than that of the colder air, the currents are rendered visible by the distortion of objects viewed through them.

Within doors, a similar example occurs above the foot-lights of the stage of a theatre; the flame of a candle, or the smoke of a lamp.

Why are the gas chandeliers in our theatres placed under a large funnel?

Because the funnel, by passing through the roof into the outer air, operates as a very powerful ventilator, the heat and smoke passing off with a large proportion of the air of the house.

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 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.