530 THE POPULAR SCIENCE MONTHLY. 



modest investigators who have occupied themselves with the chemicril 

 constitution and physical properties of sea-water. 



Sea-water, it is well known, when it is not muddy, is one of the 

 clearest of all natural waters. When we walk along the shore at low 

 tide, it is often difficult, unless we are careful, to keep from stepping 

 into the occasional pools on the rocks, the water in the little hollows 

 being so transparent as to be invisible. The question of the color of 

 this water deserves serious examination, and labors on the subject are 

 not wanting. The most notable ones are those of Father Secchi, of 

 Professor Tyndall, and the more recent researches of M. W. Spring 

 and M. Soret. 



Father Secchi made his experiments in 18G5, on board a Pontifical 

 corvette. A number of disks, formed by stretching variously- colored 

 cloths over iron hoops, the largest twelve feet in diameter, were let 

 down at a time when the conditions of the weather were most favorable 

 for transparency. The largest disk, which was painted white, became 

 invisible at the depth of about forty-two metres, while the smaller disks 

 and a delf plate, distorted by refraction, went out of sight at smaller 

 depths. The disappearance seemed to depend upon the confusion of 

 the image, which was broken up in every direction. The largest 

 disk, the considerable surface of which offered more resistance to the 

 distortion, finally ceased to be perceived, because its color, turning in 

 succession to light green, blue, and dark blue, became at last as dark 

 as the surrounding medium. Disks, painted yellow or red, were lost 

 to sight still more quickly, or under not more than twenty metres of 

 water. Repetitions of similar experiments gave co-ordinate results ; 

 and it may be stated, as a general rule of average, that the practical 

 limit of submarine vision, under favorable circumstances, is at twenty- 

 five metres under the surface. 



It was found, by spectroscopic examinations of the light reflected 

 from the differently colored disks, that the yellow was enfeebled and 

 extinguished first, and next the red, under the increasing thickness of 

 the overlying water. By the gradual disappearance of these two 

 colors, a white object is made to pass through green to blue— the tint 

 which all such objects finally assume when sunk under salt-water. 

 Each of the three simple colors — yellow, red, and blue, or violet — has 

 its distinct part among the solar rays. Yellow is luminous, red is 

 calorific, and violet-blue provokes chemical reactions. Water, in a 

 very thick mass, is neither transparent nor diathermanous ; but, 

 being penetrable to the blue, indigo, and violet rays, it is diactinic. 

 These radiations, too, will, of course, gradually lose their energy, and 

 become extinguished at last in a very deep stratum of liquid ; but the 

 limit is extremely remote. 



According to the theory propounded by Professor Tyndall, the 

 sea-waves present three principal hues — blue, green, and yellow. The 

 indigo-blue waters are the purest, while the yellow ones contain muddy 



