However, it is to be noticed that there are two ways in which fireceases to exist; it may go out either by exhaustion or byextinction. That which is self-caused we call exhaustion, that dueto its opposites extinction. [The former is that due to old age, thelatter to violence.] But either of these ways in which fire ceasesto be may be brought about by the same cause, for, when there is adeficiency of nutriment and the warmth can obtain no maintenance,the fire fails; and the reason is that the opposite, checkingdigestion, prevents the fire from being fed. But in other cases theresult is exhaustion,-when the heat accumulates excessively owing tolack of respiration and of refrigeration. For in this case whathappens is that the heat, accumulating in great quantity, quickly usesup its nutriment and consumes it all before more is sent up byevaporation. Hence not only is a smaller fire readily put out by alarge one, but of itself the candle flame is consumed when inserted ina large blaze just as is the case with any other combustible. Thereason is that the nutriment in the flame is seized by the largerone before fresh fuel can be added, for fire is ever coming into beingand rushing just like a river, but so speedily as to eludeobservation.Clearly therefore, if the bodily heat must be conserved (as isnecessary if life is to continue), there must be some way of coolingthe heat resident in the source of warmth. Take as an illustrationwhat occurs when coals are confined in a brazier. If they are keptcovered up continuously by the so-called 'choker', they are quicklyextinguished, but, if the lid is in rapid alternation lifted up andput on again they remain glowing for a long time. Banking up a firealso keeps it in, for the ashes, being porous, do not prevent thepassage of air, and again they enable it to resist extinction by thesurrounding air by means of the supply of heat which it possesses.However, we have stated in The Problems the reasons why theseoperations, namely banking up and covering up a fire, have theopposite effects (in the one case the fire goes out, in the other itcontinues alive for a considerable time).
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Everything living has soul, and it, as we have said, cannot existwithout the presence of heat in the constitution. In plants thenatural heat is sufficiently well kept alive by the aid which theirnutriment and the surrounding air supply. For the food has a coolingeffect [as it enters, just as it has in man] when first it is takenin, whereas abstinence from food produces heat and thirst. The air, ifit be motionless, becomes hot, but by the entry of food a motion isset up which lasts until digestion is completed and so cools it. Ifthe surrounding air is excessively cold owing to the time of year,there being severe frost, plants shrivel, or if, in the extremeheats of summer the moisture drawn from the ground cannot produceits cooling effect, the heat comes to an end by exhaustion. Treessuffering at such seasons are said to be blighted or star-stricken.Hence the practice of laying beneath the roots stones of certainspecies or water in pots, for the purpose of cooling the roots ofthe plants.Some animals pass their life in the water, others in the air, andtherefore these media furnish the source and means of refrigeration,water in the one case, air in the other. We must proceed-and it willrequire further application on our part-to give an account of theway and manner in which this refrigeration occurs.
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