OX THE COLORS OF WATER. 73 



liquid was entirely opaque, as much so as if it had been ink I had put 

 into the tube. I then took the mixture from the tube, diluted it to a 

 suitable degree with pure water, and introduced a current of carbonic 

 anhydride sufficient to precipitate the lime as a carbonate, and finally 

 to dissolve the carbonate as an acid carbonate of lime. The current 

 of carbonic anhydride was interrupted from time to time, and the 

 liquid was clarified and examined in the tube, and, as I did so, I could 

 see the opacity slowly disappear, letting in first a brown light, then 

 clear brown, then yellow, then green, and at last, after eighteen hours 

 of circulation of the carbonic anhydride, the liquid had again become 

 blue, but with a tendency to green. Thus, by the combined action of 

 carbonic anhydride and carbonate of lime, it is possible to produce all 

 the colors of natural waters, from opacity to greenish-blue. I reversed 

 the process, and from a green saturated solution of bicarbonate of lime 

 and carbonic acid, by gradually expelling the carbonic acid, obtained 

 a succession of colors, in inverse order, to complete opacity. Similar 

 processes with solutions of other salts gave results agreeing with 

 these. 



My experiments enabled me to verify several facts. First, we find 

 that not all of a luminous ray can pass through a considerable mass of 

 a liquid holding foreign bodies in suspension, even when the latter are 

 transparent or colorless. Further, it is not necessary that the body in 

 suspension be in the solid state. The important point is, that it be 

 competent to reflect light. Then the light - rays of feebler intensity 

 suffer extinction, one after another, according to the thickness of the 

 medium, till the yellow rays, the brightest to our eyes, are the last to 

 survive the struggle. It is not essential to the production of this phe- 

 nomenon that the medium be liquid. It may be observed in our at- 

 mosphere, where the shadow of a cloud of smoke will appear yellow or 

 brown according to the thickness of the smoke. It may be that the 

 reflecting particles can be dispensed with, and we may say generally 

 that, when light passes through an optically resistant medium, the yel- 

 low rays are the last to be extinguished. 



Other experiments have satisfied me that the yellow tint exists not 

 only when the liquids contain matter in suspension, but also when they 

 contain it in solution to the point of saturation, or when precipitation 

 is about to begin, at which point there still remains enough of this 

 color to form with the natural blue of the water a green. This con- 

 dition may be called, in analogy with the nascent cloud of Tyndall, 

 that of nascent precipitation. We now come to another view, which 

 is supported by a small number of experiments I have made with ref- 

 erence to it, that the obstruction of light, inducing the yellowish tint, 

 which is produced by any salt, depends less on the quantity of the salt 

 present than on its being near the stage of precipitation. Small quan- 

 tities of a feebly soluble salt produce the same effect as large quanti- 

 ties of a more soluble salt. The variety in the colors of natural waters 



