8o 



SCIENCE. 



manner, Count Xavier De Maistre ascribed the green 

 tints to the yellow light, which, penetrating . the water 

 and reaching the white bottom or other light-colored 

 submerged objects, and being reflected and mixed with 

 the blue which reaches the eye from all quarters, pro- 

 duces the green. (" Bibl. Univ.," Vol. 51. pp. 259-278, 

 Nov., 1832 ; also Am. J. Sci., first series, Vol. 26. pp. 65- 

 75, 1834.*). On the other hand, after Bunsen, in 1847, 

 had established that chemically pure water extinguished 

 the rays of light constituting the red end of the solar 

 spectrum more copiously than those of the blue extremity, 

 so that the transmitted tints were more or less tinged 

 with blue, some chemists were inclined to attribute the 

 green color of certain waters to the presence of foreign 

 coloring substances. Thus Bunsen himself explained the 

 brown colors of mauy waters, especially of the north- 

 German inland lakes, as produced by an admixture of 

 humus ; but he considered the green tints of the Swiss 

 lakes and silicious springs of Iceland as rising from the 

 color of the yellowish bottom. (Vide. loc. cit. ante., p. 

 44, et seq.) Similarly we find that Wittstein, in i860, 

 from chemical considerations, concluded that the green 

 color derives its origin from organic admixtures, because 

 the less organic substance a water contains the less does 

 the color differ from blue ; and with increase of organic 

 substances the blue gradually passes into green, and ul- 

 timately into brown. This is likewise the view taken in 

 1862, by Beets, for he insists that in all waters the ob- 

 served color of the liquid is that of the transmitted light, 

 and not, in any case, of the reflected light. Moreover, he 

 maintains that Newton, De Maistre, Arago and others 

 were mistaken in classifying water among those bodies 

 which have a different color by transmitted light to that 

 which they have by reflected light. (Loc. cit, ante.) 



We have already shown that if the waters were chemi- 

 cally pure and perfectly free from suspended particles, the 

 red rays of the traversing solar light would be first ab- 

 sorbed and disappear, while the other colored rays pass to 

 greater depths, one after the other being extinguished in 

 their proper order, viz., red, orange, yellow green, blue 

 and violet, until at last there is a complete extinction of 

 the light in the deeper mass of the liquid. But the pres- 

 ence of suspended particles causes a part of the traversing 

 solar light to be reflected, and according as this reflected 

 light has come from various depths, so will the color 

 vary. If, for example, the particles are large, or are 

 abundant and freely reflect from a moderate depth, and 

 prevent reflection from a greater depth, the color will be 

 some shade of green. 



When the water is shallow and a more or less light- 

 colored bottom, or a submerged object reflects the trans- 

 mitted light to the observer through the intervening stra- 

 tum of liquid, it is evident that the chromatic tints pre- 

 sented must be due to the combined influence of the se- 

 lective absorption of the water itself, and the selective re- 

 flection from the smaller suspended particles. 



In other terms, under these conditions, the tints are 

 produced by the mingling of the blue rays with the yel- 

 low, orange or red ; so that the resulting hues must gen- 

 erally be some shade of green. In short, all the facts es- 

 tablished by modern iuvestigations seem to converge and 

 point to the admixture of the blue rays reflected from the 

 smaller suspended particles with the yellow orange and 

 red rays reflected from the grosser matters below, as the 

 true physical cause of the green tints of such waters. 



The establishments of the very important function of sol- 

 id particles held in suspension in water, in producing chro- 

 matic modifications both in the scattered light and in the 

 transmitted light, serves to reconcile and to harmonize the 



♦Similarly, Aiago has very ingeniously applied the same principles to 

 the explanation of the varying colors of the waters of the ocean under 

 different circumstances, showing that when calm it must be blue by the 

 reflective light, but when ruffled the waves acting the part of prisms, 

 refract to the eye some of the transmitted light from the interior, and it 

 then appears green. (" Comptes Rendus," tomevii.,p. 219, Juiy 23d, 

 1838.) 



apparent discrepancies and contradictions in the views of 

 physicists who have investigated the color of water. 



We have already seen that Sir I. Newton and most of 

 his successors, as late as 1847, regarded water as belong- 

 , ing to the opalescent class of liquids, in which the diffuse 

 t reflected light and the transmitted light present more or 

 less complementary tints ; the former partaking more of 

 j the colors constituting the blue end of the solar spectrum, 

 while the latter presented more of the hues belonging to 

 the red extremity. On the contrary, the more recent and 

 more accurate experiments render it quite certain that in 

 distilled water the rays of the red end of the spectrum are 

 more copiously absorbed than those of the blue extrem- 

 ity; so that the emergent transmitted tint is yellowish 

 green or greenish blue. At first view, these results ap- 

 j pear to be discordant and irreconcilable ; but, it will be 

 recollected, that while even the most carefully distilled 

 water contains a sufficient amount of suspended matter, 

 to scatter enough light, to render the track of the trav- 

 ersing concentrated solar beam visible, yet, in this case, 

 the selective reflection of the blue rays, due to the suspend- 

 ed particles, is not adequate to neutralize the selective 

 molecular absorption of the rays toward the red end of the 

 I spectrum. Nevertheless, as has been previously shown, 

 ; the addition of very minute quantities of diffused suspended 

 1 matter confers on distilled water the dichroitic properties 

 i of an opalescent liquid. 



The presence of an extremely small amount of suspend- 

 j ed solid corpuscles, by selectively reflecting the shorter 

 I waves of light, is sufficient to neutralize and overcome the 

 I selectively absorbent action of the molecules of water on 

 the longer waves ; and thus, to impart yellow, orange or 

 red tints to the transmitted beam. Moreover, it is very 

 questionable whether any natural waters are sufficiently 

 i free from suspended matter to deprive them of these di- 

 I chroitic characteristics. 



Under this aspect of the subject, the views of Newton, 

 1 derived from the observations of Halley, those of Hassen- 

 I fratz deduced from his own experiments, as well as the 

 explanations of the green tints of certain waters given by 

 De Maistre, Arago and others, completely harmonize with 

 the conclusions deducible from modern researches, provi- 

 ded the property of selective reflection is transferred from 

 the aqueous molecules to the finely-divided particles held 

 in suspension. 



As a striking illustration of the slight causes which 

 sometimes transform the purest water into an opales- 

 cent or dichromatic liquid, it may be interesting to detail 

 one of my own experiences. On the 21st of Dec., 1878, 

 the series of glass tubes employed in my experiments (as 

 previously indicated), being filled with distilled water, 

 the transmitted solar beam presented when received 

 upon a white screen in a darkened room, the usual yel- 

 Lwish-green tint of my winter observations. On the 

 24th of December, or after an interval of three days, dur- 

 which all parts of the apparatus had remained in situ, I 

 was much surprised to find that the transmitted solar 

 beam was enfeebled, and presented an orange red color 

 with no tinge of green. Puzzled to discover what could 

 have produced so marked a change in the optical prop- 

 erties of the liquid, the " scientific use of the imagina- 

 tion " pictured the possible development of ultra-micro- 

 scopic germ, infusoria, bacteria, coiifcrva, etc. The 

 next day (December 25th), the same phenomenon pre- 

 sented itself, when I called the attention of my assistant, 

 Mr. August Harding, who had kindly prepared the ar- 

 rangement of the tubes, to the anomalous change that 

 had taken place in the color of the transmitted beam. 

 He suggested that as he had used alcohol in cleaning 

 the glass plates, closing the ends of the tubes, and as the 

 plates were secured to cotks by means of Canada bal- 

 sam, the alcohol absorbed by the corks, being gradually 

 diffused, dissolved some of the balsam, which solution, 

 mingling with the water, might produce a fine resinous 

 precipitate, which might stifle the transmitted beam and 



