THE HYGIENIC INFLUENCE OF PLANTS. 429 



the equator. The Germanic races, particularly, inevitably degenerate 

 after living for a few generations in the tropics, and must be con- 

 tinually renewed by immigration if they desire to retain supremacy, 

 as is proved by the case of the English in India. They will not be 

 able to settle there and maintain the characteristics which have made 

 them dominant, until means have been found of diminishing the heat 

 of the body at pleasure, as we are able to maintain it in the north. 

 At present our remedies against heat are baths, fans, and shade. 



We lose the heat of our bodies in three different ways : by the 

 medium in which we are, generally the air, and which can be warmed; 

 by the evaporation of perspiration ; and by radiation from bodies of a 

 lower temperature, not taking into account a small portion of heat 

 which goes off in mechanical labor. Under ordinary circumstances 

 in temperate climates, we lose half the heat generated by radiation, 

 one-fourth by evaporation, and one-fourth by the conducting medium 

 in which we are. In proportion as any of these methods is diminished, 

 one or both the others must be increased. As long as possible, our 

 organisms are so obliging as to open and close the sluices themselves 

 without our cognizance, provided that our regulating apparatus is in 

 order, that we are not ill. It is only when our good servant the skin, 

 under certain conditions, has come to an end of its powers, that we 

 begin to feel that we must lend our aid. And thus we have found by 

 experience that, in hot weather, shade helps the body to keep cool to 

 the needful extent. The chief effect of shelter is to prevent the sun's 

 rays from striking us directly ; but, if this were all, it would be as 

 cool in the height of summer in-doors, or even under the leaden roofs 

 of Venice, which have driven many to frenzy and desperation, as un- 

 der the shade of a tree or in a wood. It also makes a great difference 

 whether the sun's rays fall on thick foliage or on a roof of slate or 

 metal. A great deal of heat is neutralized by evaporation from the 

 leaves ; another portion by the decomposition of carbonic acid, just 

 so much as is set free when we burn the wood and other organic com- 

 binations into the composition of which it enters. The heat produced 

 by burning wood in a stove is derived from the sun ; it is but the 

 captured rays of the sun again set free by combustion. We learn 

 from Ebermayer's work that the temperature of the trees in a forest, 

 and even in the tops of them, is always lower than the air in the 

 forest. 



Besides this, shade in the open air always causes a certain draught 

 which acts as a kind of fan. All must have noticed when walking in 

 oppressive heat, when the air seems still as death, that a refreshing 

 breeze arises as soon as a cloud casts a shade. The same thing may 

 often be observed in summer in walking through a street with close 

 rows of houses, when the air is still, and one side is sunny, the other 

 in shade. On the sunny side there is not a breath of air, while on the 

 other there may be a light breeze. This is easily explained ; so far as 



