danger enter a room, the air of which is heated to 200°. Both air and liquids have heat imparted to them by a process called convection, in which heated particles, as they become lighter by expansion, rise, forming an upward current, while colder particles, being heavier, sink; but, becoming heated, rise again to give place to others. Thus a sort of circulation is kept up. Solid bodies communicate heat by conduction, because their particles, although vibrating, cannot change their material arrangement and position, the heat or vibrating motion simply passing from particle to particle, whereas the particles of liquids and gases are free to move, separate from each other, and, as Tyndall says, penetrate "in right lines through space."
I have said that substances tend to a uniform temperature with their surroundings, and that two bodies, such as a stone and a piece of cloth, will be found, other things equal, to be of the same temperature. When we come to speak of living things, however, whether vegetable or animal, although according to the laws of heat, there is the same tendency to uniformity, there is here a counteracting agency at work, and every living thing maintains a temperature peculiar to itself, and dependent on its own vital constitution, a temperature a rise only a few degrees above, or a fall only a few degrees below, which would cause its death.
During long-continued frost a thermometer in the centre of a tree trunk does not sink to freezing
