EVOLUTION OF HEAT. 631 



still more remarkable degree, between the ages of 8 and 12 years the time 

 when children are most severely affected by such epidemics. As the constitution 

 acquires greater vigor, and the bodily structure attains its full development, the 

 influence of the season upon mortality becomes less apparent; so that, at the age 

 of from 25 to 30 years, the difference between the summer and winter mortality 

 is very slight. This difference reappears, however, in a very marked degree, at 

 a later period, when the general vigor and the calorifying power undergo a 

 gradual diminution. Between the ages of 50 and 65 it is nearly as great as in 

 early infancy; and it gradually becomes more striking, until, at the age of 90 

 and upwards, the deaths in January are 158 for every 74 in July (a proportion 

 of 2 to 1); and the average of the three winter months is 145, whilst that of 

 the three summer months is only 68, or less than one-half. The results of the 

 comparisons which have now been carried out for many successive years, in the 

 Reports of the Registrar-G-eneral, between the variations in the weekly rate of 

 mortality in the metropolis and the range of atmospheric temperature, present a 

 close coincidence with the foregoing : it being especially to be noted that the rate 

 of mortality (save during the prevalence of any fatal epidemic) is almost invaria- 

 bly the highest during the winter months ; that the increase of deaths at that 

 period is most marked amongst children and old people ; and that any extraordi- 

 nary severity of winter cold constantly produces a great augmentation in the 

 mortality, the weekly number of deaths rising from, the average of 1000 (or 

 thereabouts) to 1200, when the mean temperature of the twenty-four hours falls 

 a degree or two lower than the freezing-point. 



665. Having thus considered the means by which the degree of Heat neces- 

 sary for the performance of the functions of the Human system is generated, 

 we have to inquire how its temperature is prevented from being raised too high ; 

 in other words, what frigorifying means there are, to counterbalance the influ- 

 ence of causes which, in excess, would otherwise be fatal, by raising the heat of 

 the body to an undue degree ( 654). How is it, for example, that, when a 

 person enters a room whose atmosphere is heated to one or two hundred degrees 

 above his body, the latter does not partake of the elevation, even though ex- 

 posed to the heat for some time ? Or, since the inhabitants of a climate, where 

 the thermometer averages 100 for many weeks together, are continually gene- 

 rating additional heat in their own bodies, how is it that this does not accumu- 

 late, and raise them to an undue elevation ? The means provided by Nature for 

 cooling the body when necessary, are of the simplest possible character. From 

 the whole of its soft moist surface, simple Evaporation will rake place at all 

 times, as from an inorganic body in the same circumstances ; and the amount of 

 this will be regulated merely by the condition of the atmosphere, as to warmth 

 and dryness. The more readily watery vapor can be dissolved in atmospheric 

 air, the more will be lost from the surface of the body in this manner. In cold 

 weather, very little is thus carried off", even though the air be dry : and a warm 

 atmosphere, already charged with dampness, will be nearly as ineffectual. But 

 simple evaporation is not the chief means by which the temperature of the body 

 is regulated. The Skin, as already mentioned (646), contains a large number 

 of glandulge, the office of which is to secrete an aqueous fluid; and the amount 

 of this Exhalation appears to depend solely or chiefly upon the temperature of 

 the surrounding air. Thus, when the external heat is very great, a considerable 

 amount of fluid is transuded from the skin ; and this, in evaporating, carries off 

 a large quantity of the free caloric, which would otherwise raise the temperature 

 of the body. If the atmosphere be hot and dry, and also be in motion, both 

 exhalation and evaporation go on with great rapidity. If it be cold, both are 

 checked, the former almost entirely so; but, if it be dry, some evaporation still 

 continues. On the other hand, in a hot atmosphere, saturated with moisture, 

 exhalation continues, though evaporation is almost entirely checked; and the 



