PHYSIOGRAPHY. 45 



Atmospheric- Water. 



Gaseous. Transparent. 



Sp. gr. =1-0. Nearly 800 times lighter than distilled 

 water. 



1. It is tasteless and without odor, except that of electricity, which it 

 sometimes very manifestly exhibits. Though transparent, it neverthe- 

 less reflects a blue color when in large masses, as in the sky above us. 

 The lower atmosphere is contaminated in a greater or less degree by ev- 

 ery kind of air or vapor which can be formed by the various bodies that 

 compose the earth's surface. Over the land espe<yaUy, carbonic acid is 

 mingled with it in a proportion generally equal to 0-001. 

 2. Analysis. t 



Nitrogen 79-00 



Oxygen 21-00 



8. Atmospheric-air constitutes the atmosphere, and surrounds the 

 whole globe to the height of forty or forty-five miles. 



ATMOSPHERIC WATER. P u r-e A t m o s p h e r i c- 

 Water. MOHS. 



Liquid. Transparent. 

 Sp.gr. =1-0. 



1. Its form of aggregation is continually liable to fluctuation from 

 changes of atmospheric temperature ; and instead of water there appears 

 aqueous vapor, or steam, and ice and snow. Ice is commonly produced 

 with too much rapidity to permit the separate crystals of which its mass- 

 es are composed, to be distinctly visible. They have been observed, 

 however, by SCORESBY and HERixcAtrT DE THTJRY, under the form 

 of hexahedral prisms, of which the terminal faces presented striae paral- 

 lel to the faces of the prism, and in some instances with truncated termi- 

 nal edges. The sp. gr. of ice = '92. The crystals of snow present an 

 almost endless variety of forms, which are perfect geometrical figures. 

 They are usually lamellar, and transparent ; often in regular hexagons, 

 having six points radiating from their centres, with parallel collateral 

 ramifications in the same plane : in slender six-sided needles, or spines ; 

 and in combinations of hexagons and spines, producing stelliform, or 

 wheel-shaped compositions. Dr. BREWSTER has observed quadrangu- 

 lar plates in the hoar-frost crystallized upon leaves and stones; which 



