572 RELATIONS OF HEAT. 



we make our examination in a critical manner, the problem is not so sim- 

 ple as appears at first sight, and that there are several different relations 

 of the heat which must be considered. Thus the geography of plants is 

 not wholly determined by the mean temperature of the whole year, nor 

 by the greatest heat of the summer, nor the greatest cold of winter ; that 

 is to say, it neither follows the isothermal, isotheral, nor isochimenal lines. 

 Moreover, the luxuriance of vegetation is not so much dependent upon 

 the temperature or intensity of heat as it is upon the quantity. These 

 remarks apply with much force to the case now before us, 



Intensity and jr.r / 



quantity of for the change of complexion is not so much dependent upon 

 heat compared. tte i ntens i tv O f l iea t determined by the thermometer as it is 

 upon the absolute annual quantity ; for, though these conditions of in- 

 tensity and quantity of heat are essentially distinct, yet it will generally 

 happen that they may increase or diminish together, without there being 

 an absolute correspondence between them. There can be no doubt that 

 the quantity of heat annually furnished in Guinea vastly exceeds the 

 quantity annually furnished to any part of tropical America. It is upon 

 this condition, and not upon the height of the thermometer, that the dark- 

 ening of the human complexion depends. 



To the reader who is not familiar with the technicalities of Natural 

 Philosophy, an explanatory illustration of the statement here made may 

 be of value. If he will suppose that he examines a wine-glass of water, 

 boiling hot, and a gallon of tepid water by a thermometer, he will find 

 that that instrument will stand much higher in the wine-glass than in 

 the gallon. But if he proceeds to determine how much ice the two por- 

 tions of water will" respectively melt, he will find that the greatest effect 

 is produced by the lukewarm water. We say, therefore, that though the 

 thermometer has indicated the intensity of the heat in the two portions 

 of water respectively, that is to say, their temperatures, it has not indi- 

 cated tlie quantity present in each, but the melting of the ice has revealed 

 the fact that the tepid water, by reason of its larger proportion, contains 

 a larger quantity of heat. 



It may be repeated, therefore, that the absolute quantity of heat an- 

 nually furnished to any locality is by no means -indicated by the maxi- 

 mum height to which the thermometer will rise in the summer season, 

 yet it is upon that condition, quantity, that the tint of the complexion 

 depends. 



That climate does thus influence color is clearly demonstrated by the 

 Quantity of ^ act ^at a f am ^7 f men, indisputably derived from a corn- 

 heat influences mon stock, have different complexions in different countries. 

 The Jews of the north of Europe are fair men, often having 

 red beards and blue eyes. As we trace them in their southeasterly dis- 

 tribution, their color deepens by degrees. In their original country they 



