136 DEGREES OF HEAT. 



days are short, and the few hours of mid-day sun 

 would only rouse the energies of vegetation for a 

 little, to be destroyed by the rigour of the long night. 

 But though the light and heat are thus, at those times, 

 in those places, excluded from contact with the 

 earth, and action upon its vegetable productions, they 

 are not lost. The white surface sends them up- 

 wards to warm the air ; and as there is little evapo- 

 ration there, and little vapour in the sky to absorb 

 the heat, the atmosphere maintains a far more com- 

 fortable temperature than one would be led to sup- 

 pose. Thus, in every place, and at every season, 

 there is something in nature to compensate man for 

 what the inhabitants of other countries regard as his 

 privations. 



Heat is still more wonderful than even light, won- 

 derful as that is, and abundant as are the information 

 and the pleasure which we derive from it. Like 

 light, we never can find heat alone ; for as light is 

 only perceived when something lightens or is light- 

 ened ; so we become conscious of the existence of 

 heat only when something heats or is heated. Thus, 

 as we never can by any process in nature, or any 

 experiment that we can perform artificially, obtain 

 any knowledge either of light or of heat as a dis- 

 tinct substance, or even as a material and measure- 

 able part of any substance, we cannot know any 

 thing further of either than as a property of those 

 substances in which we perceive its effects. To 

 speak of the properties either of light or heat is an 

 absurdity, because we know light and heat themselves 

 only as properties ; and therefore all their count- 

 less variations are variations only in degree ; and 

 as no property can be the measure of another prop- 

 erty in the same way that one weight is the measure 

 of other weights, or one length the measure of other 

 lengths, there is no standard to which we can bring 

 either light or heat, except we make some degree 

 of each which we find constant, as displayed in some 



