BOILING. When all other circum stances are the same, the evaporation of liquids increases with their temperature : and after they are heated to a certain tem perature, they assume the form of elastic fluids with great rapidity. If the heat be applied to the bottom of the vessel con taining the liquids, as is usually the case after the whole liquid has acquired this temperature, those particles of it which are next the bottom become an elastic fluid first : they rise up as they are form ed, through the liquid, like air bubbles, and throw the whole into violent agitation. The liquid is then said to boil. Every particular liquid has a fixed point at which this boiling commences (other things being the same ;) and this is called the boiling point of the liquid. Thus, water begins to boil when heated to 212°. It is remarkable, that afier a liquid has begun to boil, it never becomes any hot ter, however strong the fire be to which it is exposed. A strong heat indeed makes it boil more rapidly, but does not increase its temperature. This was first observ ed by Dr. Hooke. The following table contains the boiling point of a number of liquids.
It will be seen, when we come to trea: of the melting point of solids, that it is capable of being varied considerably by altering the situation of the body. Thus, water may be cooled down considerably lower than 32° without freezing. The boiling point is still less fixed, depending entirely on the degree of pressure to which the liquid to be boiled is exposed. If we diminish the pressure, the liquid boil, at a lower temperature ; if we in crease it, a higher temperature is neces sary to produce ebullition. From the ex periments of Professor Robinson, it ap pears that in a vacuum, all liquids boil about 145° lower than in the open air, under a pressure of 30 inches of mercu ry ; therefbre water would boil in va cuo at 67°, and alcohol at 34°. In a Pa pin's digester, the temperature of water may be raised to 300°, or even 400°, with out ebullition ; but the instant that this great pressure is removed, the boiling commences with prodigious violence.