CONVERSATION XXII.

Of the Thermometer.

CHARLES. Is quicksilver, when frozen, a solid metal, like iron and other metals ?

Father. It is thus far similar to them, that it is malleable, or will bear hammering. And when quicksilver boils, it goes off in vapour like boiling water, only much slower. Hence it has been inferred, that all bodies in nature are capable of existing either in a solid, fluid, or aeriform state, according to the degree of heat to which they are exposed.

Emma. I understand that water

may be either solid, as ice, or in its fluid natural state, or in a state of

vapour or steam.

But

Father. I do not wonder that you call the fluid state of water its natural state, because we are accustomed, in general, to see it so; and when it is frozen to ice, there appears to us, in this country, a violence commited upon nature. if a person, from the West or East Indies, who had never seen the effects of frost, were to arrive in Great Britain during a severe and long continued one, such as formerly congealed the surface of the Thames, unless he were told to the contrary he would conclude that ice was some mineral, and naturally solid.

Emma. Does it never freeze in the East or West Indies?

Father. It seldom freezes, unless in very elevated situations, within 35 degrees of the equator north and south it scarcely ever hails in latitudes higher than 60°. In our own climate, and indeed in all others between 35° and 60°, it rarely freezes till the sun's meridian altitude is less than 40 degrees. The coldest part of the 24 hours is generally about an hour before sun rise, and the warmest part of the day is usually between two and four o'clock in the afternoon.

Charles. Are there no degrees of heat higher than that of boiling mercury?

Father. Yes, a great many: brass will not melt till it is heated more than six times hotter than boiling mercury; and to melt cast iron re

quires a heat more than six times greater than this.

Emma. By what kind of thermometer are these degrees of heat measured?

Father. The ingenious Mr. Wedgewood has invented a thermometer for measuring the degrees of heat up to 32277° of Fahrenheit's scale.

Charles. Can you explain the structure of this thermometer?

Father. All argillaceous bodies, or bodies made of clay, are dimiuished in bulk by the application of great heat. The diminution com

mences in a dull red heat, and proceeds regularly as the heat increases till the clay is vitrified, or transformed into a glassy substance. This is the principle of Mr. Wedgewood's ther

mometer.

Emma. Is vitrification the limit of this thermometer?

Father. Certainly. The construction and application of this instrument are extremely simple, and it marks all the different degrees of ignition, from the red heat, visible only in the dark, to the heat of an air furnace. It consists of two rulers fixed on a plane, a little farther asunder at one end than at the other, leaving a space between them. Small pieces of alum and clay, mixed together, are made just large enough to enter at the wide end: they are then heated in the fire with the body whose heat is to be ascertained. The fire, according to its heat, contracts the earthy body, so that, being applied to the wide end of the gauge, it will slide on towards the narrow