70 THE POPULAR SCIENCE MONTHLY. 



minute particles similar to those found by Tyndall in the atmosphere. 

 Hagenbach repeated the experiments, with like results, in the Lake of 

 Lucerne ; and Tyndall, a year later, with water of the Mediterranean 

 and the Lake of Geneva, sent to him in London. Mr. Hayes ex- 

 amined the water of the Lake of Geneva, to see if it did not contain a 

 oloring substance, and found none. 



The later of these experiments indicate that, contrary to Bunsen's 

 belief, water by itself may be colorless, but nothing is less certain. M. 

 Soret says that the lake was still blue in cloudy weather, when he 

 could not get a trace of polarization effects. Is this not enough to 

 prove that reflection is not the only cause of color in water ? More- 

 over, if the blue in water were wholly of the same origin as that of the 

 sky, the light transmitted by water should be crimson, as that which is 

 shown on the tops of high mountains, or which is transmitted through 

 the clouds at the rising and setting of the sun ; but nothing of the 

 kind is the case. Professor Tyndall states this, and Father Secchi has 

 established the absence of the red and yellow from the absorption spec- 

 trum of water. It is also well known to those who have had occasion 

 to make submarine excursions, or who have visited the glacial grottoes 

 in Switzerland, that the transmitted light has a blue tone, and the red 

 is so weak that the figures have a livid aspect. 



These facts show that the question is still waiting a definite solu- 

 tion. We now turn to the explanations which have been offered of 

 the diversities in the colors of natural waters. 



According to Arago, water has two colors, " a color of transmission 

 and a color of reflection, wholly different from the other. It appears 

 blue by reflection, and its transmitted color is green." This suppo- 

 sition can not be reconciled with optical laws, but Arago used it to 

 explain the variations of tint in the water of a shallow, white-bottomed 

 sea. " When the sea is deep, light is reflected from the water and 

 appears blue ; but, if it is not very deep, the sand at the bottom re- 

 ceives the light through a stratum of water. The light then reaches 

 the bottom, already green, and, in returning from the sand to the air, 

 the green color is deepened, frequently so much as to predominate, on 

 coming out, over the blue. This, probably, is the whole secret of 

 those shades which are in calm weather the sure and valuable index 

 to the experienced sailor of the depth of the bottom. This explanation 

 fails when it is applied to other quarters than those for which Arago 

 conceived it. The Swiss lakes are green, or blue, independently of 

 their depth. Arago suggests, after Davy, that the change from blue 

 to green may be caused by the presence of vegetable, M. Durocher of 

 colored, matters. These suppositions are gratuitous, and supported by 

 no evidence. II. Sainte-Claire Deville, in 1848, analyzed a number of 

 natural waters, and found that the blue ones gave hardly perceptible 

 colored residues, while the green ones yielded such considerable quan- 

 tities of organic matter that the soluble salts became yellow after 



