174 MEMOmS OF THE NATIONAL ACADEMY OF SCIENCES. 



Early in the iuvestigation I was led to believe that the behavior of the suspended i»artieles of 

 solids was diffi'rent in the presence of crystalloid substances, as a chiss, from that in the presence 

 of colloids, Init at the present state of the investigation this is not proven. 



The statements of some investigators lead to the inference that the rapidity of sedimentation 

 is correlated with the activity of the "Brownian movements," and I have .studied this part of the 

 subject with es|)ecial interest. 



The rapidity of the subsidence of the particles visible witli the microscope appears to be related 

 to this, and those substances which retard the '-Brownian inovcnients" hastens the precipitation. 

 This applies to the tine grains of sand, tiocculent clays, and such inert substances as pulverized 

 charcoal. But I do not believe that the " Brownian movements" are continuously active in a liquid 

 otherwise at rest, kept in a dark, (piiet place, and where the changes of temperature are slight 

 and take place very slowly. The conditions where these movements can be observed are neces- 

 .sarily those where various forms of radiant energy are manifest, and these are the probable 

 active cause of the movements. It seems to me that where suspended clays are kept in the dark, 

 and where there is little variation in temperature, we can hardly look to the energy of the 

 "Brownian movements" as the active cause of the resistance to gravity and suspension of the 

 heavier solid in the lighter liquid. 



The later opalescence, which lingers so long, as well indeed as much of the more visible clay 

 earlier, does not exist in the liquid as separately visible particles even with the microscope. ^ All 

 the microscoi)ically visible particles settle relatively early in the experiment; the later opalescence 

 is from nltramicrosco]>ic materials. This is more visible in the suidight than in ordinary dif- 

 fused light, and shows the path of a beam of sunlight through it very strongly when the amount 

 suspended is very slight indeed. The clearest natural waters, I have found, are not optically pure, 

 but some aiipear to be without the o])alescence that I have described from suspended clays. 



Those portions of clay that remain long suspended, and are without separately visible parti- 

 cles, seem to me to be in a condition analogous to that of a colloid, reminding one of diluted 

 gelatinous silica or diluted boiled starch. In many respects the behavior of that portion of a clay 

 which will not set'.le to the bottom of a vessel a few inches deep in several days of quiet is 

 that of a colloid. Prof. S. W. Johnson, whose knowledge of soils in their chemical and agricult- 

 ural relations is .so extensive, and who has been acquainted with these experiments during their 

 progress, first suggested to me that the phenomena might be essentially chemical and to relate to 

 the state of hydration of the clays. 



Following this up, it seems to me probable that there may exist a series of hydrous silicates 

 of alumina and iron, holding very feebly different amounts of water, and having difl'erent prop- 

 erties, so far as their relation to water is concerned, .some swelling up in water more than others, 

 and diffusible in it (as colloids) with difl'erent degrees of facility, and that acids, salts, heat, and 

 other conditions change these states of hydration, and thus change the behavior of the su.spended 

 material towards water ; that some which exist in pure water at one temperature are destroyed by 

 another temperature, or by acids, salts, &c. 



There are many indications that this is the case. The use of lime in agriculture for drying 

 heavy, wet, clayey lands was known long before underdraining was extensively practiced. The 

 burning of clays and other operations in agriculture may ha\e their practical basis in the same 

 chemical properties of clay. The tendency of many, and indeed most, clays when diffused in water 

 to arrange them.selves in layers or strata in the liquid suggests the same thing, these different strata 

 representing different chemical compounds of a series. With some of the finer sedimentary clays it 

 is not uncommon to have as many as six or eight such layers in the turbid liquid, and these probably 

 represent different weak chemical compounds, settling with different degrees of rapidity, and more 



