252

METALS.

prevents the air from coming in contact with it.Strictly speaking, however, the word rust appliesonly to the oxyd, which forms on the surface ofirbn, when exposed to air and moisture, whichoxyd appears to be united with a small portionof carbonic acid.

EMILY.

When metals oxvdate from the atmospherewithout an elevation of temperature, some lightand heat, I suppose, must be disengaged, thoughnot in sufficient quantities to be sensible.

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MRS. B.

Undoubtedly; and, indeed, it is not surprisingthat in this case the light and heat should not besensible, when you consider how extremely slow,and, indeed, how' imperfectly, most metals oxy-date by mere exposure to the atmosphere. Forthe quantity of oxygen with which metals arecapable of combining, generally depends upontheir temperature; and the absorption stops atvarious points of oxydation, according to the de-gree to which their temperature is raised.

EMILY.

That seems very natural; for the greater thequantity of caloric introduced into a metal, thefurther its particles are separated from one ano-ther, and the more easily, therefore, can the}- at-tiaci the oxygen and combine with it.