166 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



opacity to the top, but will rather be disposed in successive layers, the limits of each more or less 

 well defined. 



Different specimens of clays and soils behave quite unlike in this respect, so far as the details 

 go. A few do not show these strata at all, the suspended matter fiidiug gradually and regularly 

 in density, and such clays usually settle comparatively rapidly. Others show the character but 

 feebly; there may be but two such layers, or, if more, the limits of each may be very illy deflued. 

 Some show as many as six or eight, or even more, in which cases they are of unequal thickness, 

 and sometimes the limits of each are surprisingly distinct. 



If left perfectly quiet, the heavier go down first, and at last all are down but one, and the 

 liquid is then uniformly opalescent from bottom to top. This may be after a week or two or it 

 may be only after many months. With fui-ther quiet this opalescense gradually fades evenly 

 from top to bottom, the rapidity of this clearing of the liquid being modified by several external 

 conditions. 



The fading of this opalescense by subsidence goes on slower in the light than in the dark, but 

 how nearly this is related to changes of temperature I have not been able to determine, as any 

 Ijlace at my command from which light is excluded is subject to less fluctuation of temperature 

 than where it is abundant. The nearest approach to similar conditions other than light has been 

 in an instrument closet in my lecture-room, built against a firm interior brick wall, the upper 

 part of the case being closed by glass and the lower part by wooden doors. When similar specimens 

 have been exposed in these two cases at the same time, the fading of the opalescense by subsid- 

 ence has been more rapid in the dark case than in the adjacent light one. 



If the experiment goes on in a place of perfect quiet and in the dark, and where the daily 

 changes of temperature are slight and very gradual, then the opalescense gradually fades with 

 time; but how long before it will entirely fade away and the liquid become clear by the subsidence 

 of the particles (if indeed it ever will) I cannot say, but certainly not in six years, some of my 

 samples having now stood longer than that time. 



If, however, the experiment is conducted in an ordinary lighted room, with the fluctuations 

 of temperature incident to habitation or use, then after a time the opalescense ceases to fade ; it 

 may remain stationary as to intensity, or it may increase and diminish with the seasons and other 

 fluctuating conditions. How much such fluctuation in opalescense is owing to convection cur- 

 rents produced by the fluctuations of temperature, and how much to other causes, I have found no 

 means to determine. 



The color of this ultimate uniform opalescense is usually milky, but with some ferruginous 

 clays it is red, brown, or of different shades of amber; with certain other clays it has various 

 shades of pale green and yellowish green. This is strikingly the case with certain clays from the 

 bad lands of Wyoming, the shades of color of which remind one of the tints of some of the Swiss 

 lakes as seen from the alpine heights above them, which tints and colors I suspect may be due to 

 a similar cause. 



Temperatures above that found naturally in the free air and also below the freezing point 

 are each accompanied with their special phenomena. 



The boiling of clays in a great excess of water tends to reduce them to the finest division, 

 and this is the method employed by Professor Hilgard in his elaboi-ate and most instructive in- 

 vestigations on the physical conditions and composition of soils. Some clays go to pieces easily 

 on boiling, while others, according to this authority, hav^e to be boiled for many hours, it may be 

 for days (and with precautions to prevent flocculation), before the process is complete. I find 



