14 BEYOND THE LIMITS OF VISION. 



ful marine and ultra-marine blue, and then into a milky and 

 opaque white. For resin, though perfectly soluble in alcohol, is 

 insoluble in water ; and by carefully mixing them as we have 

 described, the particles of the gum are first in a state of molecu- 

 lar division, in which they are totally invisible ; then by grad- 

 ual aggregation they become large enough to chip off and scatter 

 some of the blue rays of light, which are those of shortest 

 vibration ; and finally they become sufficiently large to intercept 

 all light. This is the explanation of the colors of deep waters, 

 verging from blue to dark green. 



Again, there are certain gaseous compounds which sunlight is 

 competent to gradually decompose into their original elements. 

 One is sulphurous acid gas, composed of two atoms of oxygen 

 and one of sulphur. It is the pungent gas one smells when a 

 sulphur match is lighted. A beam of sunlight passing through 

 a glass jar containing this gas, literally shakes the atoms apart, 

 leaving the oxygen as a pure transparent gas, and the solid sulphur 

 atoms suspended in it. For about two minutes no change of 

 color is seen ; after that time the sulphur atoms will have joined 

 together into sufficiently large particles to affect the blue rays of 

 light. Then for fifteen minutes the color of the gas is a grad- 

 ually deepening blue, and finally a thick opaque white. This 

 exemplifies in a beautiful manner the cause of the deep azure 

 blue of our clear skies. There are scattered, through all the 

 regions of the air, minute particles of matter, small in comparison 

 to the length of light waves, which deflect and scatter some of 

 the smaller and less energetic undulations of the sun-beams ; 

 and this light, left behind in the swift passage of the solar rays, 

 remains as the ever-blue firmament of the skies. 



That this light-scattering dust is mainly organic, that is, the 

 product in some way of living things, is shown by the fact that 

 it can all be burned out of any particular enclosure, and the air 

 therein be left perfectly light pure. If sunlight is passed through 

 a small apperture into a dark room, its passage through the air 

 will be marked by a white streak, from reflection against the 

 particles of matter which it meets. But if a platinum wire be 

 raised to a white heat in this room by the electric battery, and 



