378 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



due to various modifications and admixtures of reflected light from sus- 

 pended materials and from the bottom. Le Conte says : 



The scattering of the light is not molecular, but is evidently due to the presence of 

 finely divided matter in a state of suspension, whereby the shorter rays of the beam 

 are intercepted and diffused more copiously than the larger ones, thus rendering the 

 trace of the light visible in the liquid and imparting a blue tint to the laterally scat- 

 tered polarized light. The conclusion seems, therefore, to be inevitable that if wat^r 

 were perfectly free from all foreign materials, either in solutiou or chemical suspen- 

 sion — both chemically and optically pure — it would have no color at all by diffusion 

 of light; in fact, inasmuch as no scattered light would be emitted from the traversing 

 beam it would show the darkness of true transparency. 



He thinks, therefore, th'd,tpure water has no color by diftusive reflection. 



Some of the Welsh tarns or lakes, said to be bottomless, have an inky 

 hue, and, according to Tyndall,* in some places in the sea the water, 

 when looked down upon, was almost inky black, qualified by traces of 

 indigo; and under the moraines of the Swiss Alps, where the ice is ex- 

 ceptionally compact and there is no scattering of light, the perfectly 

 clear ice presents pitchy blackness. 



Bunsen was probably the first one to make direct experiments upon 

 the color of water.t He experimented with a 2-iuch tube 2 yards long 

 blackened with lampblack and wax to within an inch of the end, and 

 held against white porcelain illuminated by a white light. He says, 

 "Chemically pure water is not colorless, but has a j)ure bluish tint, 

 which is visible only when a stratum of considerable depth is pene- 

 trated by the light." The smallest quantity of foreign matter disguises 

 or alters the color. He considers that the greenish tint in the Swiss 

 lakes is due to the color of the bottom. 



Tyndall, in 1857, confirmed Bunsen's results. He used a tin tube 15 

 feet long and 3 inches in diameter, and stopped both ends. This tube 

 was half filled, so that there was a column of air as well as of water. A 

 white paper was placed a short distance from the tube upon which the 

 color was displayed. Blue-green was the purest color he got. By the 

 passage of an electric beam through distilled water he obtained a blue- 

 green image. He says : 



Water absorbs all the extra red rays of the sun, and if the layer be thick enough it 

 invades the red rays themselves. Thus, the greater the distance the solar beams travel 

 through pure water the more they are deprived of those components which lie at the 

 red end of the spectrum. The consequence is that the light finally transmitted by 

 water, and which gives it its color, is blue. 



In December, 1861, W. Beetz, of Erlangen, obtained results similar 

 to those of Bunsen and Tyndall.| Le Conte used three glass tubes 3 

 centimeters in diameter, and connected them by rubber tubing, getting 

 an aggregate length of 5 meters. He used solar light thrown into a 

 darkened chamber by a porte-lumiere. He also passed it through a 

 secondary screen, rendering it uniform in size, so that he got an approx- 

 imate mathematical beam, and secured its transmission along the axis 

 without danger of mixing with reflected light from the tube. Distilled 

 water gave him a greenish-blue or yellowish-green tint. Pacific Ocean 

 water gave green to yellowish-orange color, and the same tints were 

 found in the purest hydrant water. He says : 



Hence it appears that, in a general way, my experiments confirm the opinion that 

 pure water absorbs to a somewhat greater extent the solar rays constituting the red 

 end of the spectrum. 



* Hours of Exercise in the Alps. Voyage to Algeria to observe the Eclipse. Am. ed. 

 N. Y., 1871., pp. 463-470. 



t Edinburgh Philosophical Journal, vol. xlvii, 1849, pp. 95-98. Also, Ann. der Cb.em. 

 and Pharm., vol. Ixii, pp. 44,45, 1847. 



t Philosophical Magazine, 4th series, vol. xxiv, pp. 218-224, September, 1862. Also 

 Pogg. Ann., vol. cxv, pp. 137-147, January, 1862. 



