Clearly therefore, if the bodily heat must be conserved (as is

necessary if life is to continue), there must be some way of cooling

the heat resident in the source of warmth. Take as an illustration

what occurs when coals are confined in a brazier. If they are kept

covered up continuously by the so-called 'choker', they are quickly

extinguished, but, if the lid is in rapid alternation lifted up and

put on again they remain glowing for a long time. Banking up a fire

also keeps it in, for the ashes, being porous, do not prevent the

passage of air, and again they enable it to resist extinction by the

surrounding air by means of the supply of heat which it possesses.

However, we have stated in The Problems the reasons why these

operations, namely banking up and covering up a fire, have the

opposite effects (in the one case the fire goes out, in the other it

continues alive for a considerable time).



6



Everything living has soul, and it, as we have said, cannot exist

without the presence of heat in the constitution. In plants the

natural heat is sufficiently well kept alive by the aid which their

nutriment and the surrounding air supply. For the food has a cooling

effect [as it enters, just as it has in man] when first it is taken

in, whereas abstinence from food produces heat and thirst. The air, if

it be motionless, becomes hot, but by the entry of food a motion is

set up which lasts until digestion is completed and so cools it. If

the surrounding air is excessively cold owing to the time of year,

there being severe frost, plants shrivel, or if, in the extreme

heats of summer the moisture drawn from the ground cannot produce

its cooling effect, the heat comes to an end by exhaustion. Trees

suffering at such seasons are said to be blighted or star-stricken.

Hence the practice of laying beneath the roots stones of certain

species or water in pots, for the purpose of cooling the roots of