COMMON THINGS AIR. 



finger being applied at the upper end to stop it, it be raised, the 

 liquid which will be suspended in the tube will appear as 

 transparent and colourless as water. It cannot be doubted, 

 nevertheless, that the liquid in the tube has the same colour as 

 the liquid in the glass. The colour is not perceived only because 

 the quantity in the tube is too small to reflect sufficient colour to 

 produce a sensible effect on the eye. 



The atmosphere is in the same circumstances. The colour 

 reflected even from a considerable volume of it is too faint to be 

 perceptible. Thus the air which fills a room, or which intervenes 

 between the eye and the buildings, trees, and other objects 

 around us, appears quite transparent and colourless, and we see 

 all such objects distinctly through it in their proper colours. 

 But when, in the daytime, we look up through fifty or sixty 

 miles height of air, illuminated by solar light, we find that a 

 strong and decided tint of blue is perceived. This azure, which 

 in the absence of clouds forms the celestial vault, belongs not to 

 anything which occupies the regions of the universe in which the 

 heavenly bodies are placed, but to the vast mass of air through 

 which these bodies are seen. 



To perceive this peculiar colour of air, however, it is not 

 necessary that so vast a mass should be presented to the eye. 

 Distant mountains appear bluish, not because that is their colour, 

 but because it is the hue of the aerial medium through which we 

 look at them. As we approach them, the quantity of the inter- 

 vening air being diminished, this bluish tint is no longer perceived, 

 and they appear with their proper colours. 



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