In musts, which spontaneously ferment, the external air excites in their oils an agitation, which, heating and opening the pores of the liquor, expands and puts in action the internal air they possess. The case is not exactly the same with regard to those musts which require ferments. The air wanted in boiled worts must be supplied by the means of yeast. Was the heat of the wort such, as to occasion the immediate bursting of all the air bubbles contained in the yeast, an effervescence rather than a fermentation would ensue. Now a heat superior to 80 degrees has this effect, and is therefore one of the boundaries in artificial fermentation; 40 degrees of heat, for want of being sufficient to free the air inclosed in the yeast bubbles, and to excite their action, is the other. Within these limits, must the wort be cooled to; and the precise degree, which varies according to the different circumstances they are in, and to the intent they are to be applied to, is, together with the means of procuring this heat, the purport of this section. Worts, when in the copper, boil at a heat somewhat superior to that of 212 degrees; the more this is exceeded, the stronger the liquor is. The instant the wort is suffered to go out of the copper, it loses more heat than in any other equal space of time after it has been exposed to the air. In the course of the natural day, or in 24 hours, the heat of the air varies sometimes, (especially in summer) as much as 20 degrees. If the wort, after having reached the lowest heat in this interval, was suffered to remain in the coolers, till the return of a greater in the air, it would be influenced by this increase, expand, and be put in action; and, should there be at this time any elastic air in any part of the coolers, which sometimes happens, either from the sediment of former worts, from the backs not being clean swept, or from the wood being old and spungy, the wort supposed to be left to cool, will, by receiving the additional heat from the air, and blending with the incidental elastic air adhering to the coolers, bring on, in a lower degree, the act of fermentation; an accident by the artist called the _backs being set_. For this reason, a wort should never be suffered to lay so long as to be exposed to the hazard of this injury, which generally may happen in somewhat more than twelve hours. Thus are we directed to spread or lay our worts so thin in the backs, as they may come to their due temperature within this space; in summer it is sufficient if the backs be covered; in winter a depth of two inches may oftentimes be allowed with safety.