THE SUBSIDENCE OF PARTICLES IN LIQUIDS. 173 



inuriiitic acid for, twelve sucwessive dilutious and until the proportion of acid was by calculation 

 almost infinitesimal (one part in several thousand of the liquid) one or two days was amply suffi- 

 cient for clearing; the thirteenth cleared in nine days; the fourteenth in fourteen weeks; the fif- 

 teenth, after standing two and a half years, is still very obviously opalescent. 



Two others are exhibited in which the behavior was similar, oidy the degree of dilution was 

 not so great. One lias stood thirty and the other thirty-two niontlis, and the opalescence in each 

 case is more marked than with companion portions which were begun in pure water only. In 

 each case the precipitation was comijaratively rapid until the proportion of acid amounted to less 

 than one i)art in a thousand of the liquid, and in some the final long-continued susi)ension was 

 only reached when the amount was very much less. 



lu some of the samples iridescent films or spicules have formed in the liquid, very like silica 

 separated from solutions of dissolved glass; but the glass of the flasks used purported to be hard 

 cheniical glass, and I cannot test the samples without ajiitating tliem and thus bringing the exi)er- 

 imeuts to a close. Similar glasses similarly treated have not become decomposed, and at present 

 I think that the iridescent material is silica derived, not from the decomposition of the glass, but 

 rather from a portion of the clay which may have undergone similar decomposition. 



In the courseof the experiments with acid solutions, as they reached a suflicientdegiee of dilu- 

 tion of acid and purity of water, there were the same picking up and suspension of material that 

 was observed in the experiments with salt. 



Caustic alkalies, i)otash, soda, and ammonia have also been used. Some authorities have 

 stated that while acids and salts hasten sedimentation, alkalies check it and may even susjiend it 

 indefinitely. My experiments do not confirm this, but show rather the contrary efiect. For 

 example, the same Hartford clay which was used in experiments with salts and acids was used 

 also with alkalies. A single illustration will suffice. Beginning with a solution containing one- 

 half of 1 per cent, of pure caustic potash, the sedimentation was very much more rapid than in a 

 similar portion in pure water, but not so rapid as samples with acids or salts. Operating on weaker 

 and weaker solutions by successive decantations as before described, it would become nearly clear 

 in twelve hours, when the proportion of potash in the solution was only .0008. When it was 

 reduced to .0002 the behavior was -then much as in ])ure water, and the sample is not yet entirely 

 clear, after more than two years' standing. 



So far as my experiments go, tine clays are precipitated from alkaline solutions more quickly 

 and more completely than from pure water, no matter how dilute the solution. 



When very fine soils, rich in organic matter (as, for example, some of the rich prairie bottom 

 lands of Illinois), are agitated with water, there is much tendency, if the vessels are ke])t in the 

 light, for a growth of confervoid algic year after year, the decaying remains of which, difiused 

 through the water, go down mostly with the clay, if agitated; but the solutions never become clear 

 of the very light flocculent organic matter from the decaying alga', now rising and now falling 

 with changes of temperature and other conditions. Fresh agitation and settling may render it 

 nearly clear for a time, but a new crop will spring up, its decayed remains to behave in the same 

 way. 



In experiments with neutral organic substances — crystalloids, like cane-sugar, and colloids, 

 like extract of peat — there has been no law deduced; some hasten precipitation, others retard it. 

 still others seem indifferent. I have experimented on the sugars along with yeast to observe the 

 relations to fermentation, but this opens up a new set of reactions which I will not discuss here, 

 the phenomena varying with the intensity of the action. 



