May 8, 1896.] 



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the red and yellow rays, but reflecting 

 the rays of shorter wave-length, i. e., the 

 blue and violet. Hence, light reaching the 

 observer by transmission appears greenish, 

 the original blue of the water being added 

 to the transmitted orange rays. The blue 

 of reflection is largely absorbed, or but 

 slightly augments the color of the water. 

 Blue water should contain no turbid ele- 

 ments, but deep water absolutely free from 

 turbidity should absorb all rays and appear 

 black. Contrary to this, the Mediterranean 

 and the Lake of Geneva in their deeper por- 

 tions are intensely blue. Hence, even the 

 most limpid waters are not optically void. 

 Tyndall and Soret believed that even the 

 purest water might contain particles in 

 suspension which account for the illumina- 

 tion of the blue waters in nature. This is 

 contradicted by the evidence from experi- 

 ments with polarized light, and by the fact 

 of the absorption of the rays of minimum 

 wave-length by a turbid medium. 



As a farther test, however, Prof. Spring 

 has prepared an elaborate apparatus to 

 prove whether loss of illumination by ab- 

 sorption through a great thickness of water 

 be accompanied by a loss of transparency due 

 to the presence of particles in suspension, 

 as in the case of the atmosphere. A tube 

 of glass was constructed, 26 meters long and 

 of 15 mm. internal diameter, mounted at a 

 slight inclination on a scaffold and straight- 

 ened with hand vises until its axis co- 

 incided with the optical axis of a telescope 

 adjusted at one end. Heavy black paper 

 covered the tube throughout its length, and 

 the ends were sealed with glass plates, the 

 one nearer to the source of light bearing 

 ' cross-hairs. ' Glass tubes were fitted at 

 each end for the introduction of the water, 

 which was distilled with the utmost care in 

 platinum retorts. Either daylight or the 

 Auer incandescent burner could be used as 

 illuminants. 



A column of water 26 m. long appeared 



deep blue; with the illumination of the 

 Auer lamp, the telescope revealed the cross- 

 hairs as sharply defined as though the tube 

 were empty ; hence the presence of foreign 

 particles is improbable. Small apertures 

 pierced in the paper wall of the tube gave 

 evidence of emission of light laterally only 

 at a distance of 2 meters from the lamp. 

 This suggested some influence of the heat 

 rays; to test this, water 12° C. warmer than 

 the tube was introduced and produced com- 

 plete opacity, vrhich gradually passed off" as 

 the water regained the normal temperature. 

 The minimum difference of mean tempera- 

 ture which would produce opacity was de- 

 termined to be only 0.57° C. 



A shorter tube 6 m. long was constructed 

 of metal with a view to testing the local ap- 

 plication of heat. On applying a flame at 

 one point in the side of the tube, the 

 sharply deflned opening at the farther end 

 appeared to enlarge, became blurred and 

 finally disappeared, leaving an evenly illu- 

 mined field. The elfect suggested a cloud 

 passing before the sun. Continued applica- 

 tion of heat produced complete darkness. 



These experiments show that bodies of 

 water are not optically homogeneous when 

 traversed -by convectional currents, even 

 though the latter are caused by very slight 

 difierences in temperature : the warmer 

 portions have the properties of a turbid 

 medium. Hence less absorption and greater 

 emission of light, making apparent the blue 

 color of the water. Cooling increases trans- 

 parency, hence the differences of color pro- 

 duced by the cooling shadow on the water's 

 surface, of a cloud or a mountain ; a dry 

 wind, cooling the water's surface by in- 

 creased evaporation, will at the same time 

 increase its transparency. Conformable to 

 this explanation is the fact stated by Forel, 

 that fresh water lakes are more transparent 

 in winter than in summer. The facts de- 

 scribed are not held by the author to pre- 

 clude the operation of other agencies, for 



