116 



KANSAS Academy of Science. 



of the less refrangible rays. Many attempts have been made to fix upon the pre- 

 cise medium to which this absorption may be due. 



The fact that water is always present in our atmosphere, in a state of fine sub- 

 division, has naturally led philosophers to look to it as the absorbing medium. The 

 establishment of this theory depends, however, upon the ability to show that sun- 

 light, when viewed through a sufficiently thick layer of water, appears blue; but the 

 very numerous experiments upon this subject have given a great variety of results. 

 Many waters, notably those of Lake Geneva and of the deep sea, have this blue color 

 to a marked degree. Others, when viewed by transmitted light, appear green, yel- 

 lowish, and even brown. The presence of impurities, even in almost inconceivably 

 minute quantities, has been found to produce striking changes of color. Thus water 

 newly distilled is blue, but, upon standing even for a few hours in perfectly clean 

 vessels, it becomes green, yellow, and even brown; changes which have been traced 

 to the presence in increasing numbers of microscopic organisms, so minute and 

 so transparent as to test the microscope in the severest manner. Loret, who has 

 studied this subject exhaustively, in the hope of solving the problem of the color of 

 the water of Lake Geneva, finds that otherwise pure water containing silica produces 

 absorption blues, to which he ascribes the blue color of newly-distilled water. ^ The 



|H DBAJ p [jf 



Flu. 2.^ 



addition of other impurities, especially iron, such as give a yellow instead of a white 

 residue upon evaporation, convert this blue into green, and ultimately into brown. 

 The presence of minute quantities of silica in the water of the atmosphere has led 

 him to ascribe the color of the sky to the presence of this element in the water float- 

 ing in the air. 



The gaseous components of our atmosphere have also been looked to as the ab- 

 sorbing media to which we owe the color of the sky, and one of these — ozone- — 

 having been recently found to possess an absorption spectrum rich in violet rays, 

 ozone has been declared the substance to which the azure is due. 



The long i)revalent opinion that the atmosphere, taken as a whole, transmits the 

 violet rays more readily than those of longer wave-length, has been controverted by 

 the investigations of our countryman. Professor Langley, who brought to bear upon 

 this very difficult problem a new instrument of his own invention, an apparatus of 

 hitherto undreamed of delicacy. Langley's instrument — the bolometer — makes it 

 possible to measure radiant energy accurately by electrical means, even when the 

 rays are so feeble as to be quite out of the reach of the most sensitive of the former 

 apparatus devised for this purpose. Scarcely exceeded in delicacy by the eye itself, 

 the bolometer is not confined like the eye to impressions from the very limited 



» See E. Ray Lancaster, Nature. Vol. II, p. 235. 



» From Langley's Monograph ; Researches on Solar Heat and its Absorption by the Earth's At- 

 mosphere : Professional Papers of the .Signal .Service, No. 15. 



