Translation o/" Key's Essays. 55 



in immediate contact with it, is lighter than it was in the morn- 

 ing? just the reverse; it is much thicker, and consequently 

 heavier ; for how can the heat have subtilized the air, without 

 having caused it to ascend, and how have caused it to ascend 

 without the descent of a heavier ? Nothing rises by its own 

 power, it is the falling- down of another body which pushes it 

 up. Doubtless it has separated the most subtle, and raised it 

 on high, leaving the heavier below, as happens in the distilla- 

 tion of liquors. But if this argument do not convince you of 

 the thickening and increased weight of this heated air, at least 

 believe the evidence of your own senses. I undertake to make 

 you feel that it is so by the touch of your hand, and see it with 

 your eyes. Now, whilst it is mid-day, do you not feel the air 

 warmer to the touch, than it was a quarter of an hour after sun- 

 rise ? It is not however that the sun has imparted a higher de- 

 gree of heat to it, since that never varies, and it disperses it at 

 a constant rate within the sphere of its activity ; for in like man- 

 ner as its rays transpierced the air without resistance, and com- 

 municated all its light to it in a moment, so did it communi- 

 cate its heat, which has not itself increased, though its action 

 is greater, from the greater density of the matter it acts 

 on; for the more subtle parts being gradually raised up, 

 the others remain below in greater number, and more 

 united in the self-same place ; and from this greater union 

 comes this greater action. This may be made clearer by 

 considering the nature of elementary fire, which although hot 

 in the utmost degree, nevertheless does not burn, in conse- 

 quence of its extreme rarity, but red-hot iron burns violently, 

 not that it is hotter (for how can its heat exceed the utmost de- 

 gree) but because it is denser, containing more parts in an equal 

 space. So much for the touch, let us proceed to the other 

 sense. When the sun began this morning to send its rays on 

 the horizon, the air, by its subtlety, was wholly invisible ; but 

 now do you not see how it trembles over the furrows } It is 

 because it is thicker, and has acquired more body that it is in 

 some measure visible to you. Thus, I think, I have sufficiently 

 acquitted myself of my promise ; but it is fit that 1 should go a 



