168 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In solutions of various kinds, the pbeuomena are very different, and experiments have been 

 conducted witli various acids, alkalies, salts, extracts, and neutral organic substances. 



Some of the more general facts are well known and widely applied. The use of certain salts 

 to clear turbid waters, and of organic substances to correct unwholesome waters, are of wide appli- 

 cation, and have been known from antiquity. (See Exodus, xv, 23.) Alum is used in many coun- 

 tries where the drinking water is turbid. I have often heard of its use in the Mississijipi Basin; 

 also in South America and Euroi)e; and Mi-. Arnold Hague tells me that he found it in universal 

 use in the Loess country of Northern China, where it has been used for this purpose for ages. 



The rapidity of sedimentation in the presence of certain salts and acids, as contrasted with the 

 behavior of the same material in fresh water, is indeed striking. I exhibit one specimen to the 

 academy especially notable in this character, and which has been used to illustrate the general fact 

 to my classes. The original is a very hard, greenish, eocene clay, from the Niobrara region, which 

 when ground ui) in pure water settles very slowly. For class-room illustration a quantity which has 

 stood some days, for the coarser ])arts to settle, is decanted and divided into two equal jiortions at 

 the beginning of the lecture. To one a solution of common salt is added, to the other an equal 

 volume of distilled water, that the opacity of the two be equal at the start. Before the close of 

 the lecture the upper part of the one will be cleai-, or nearly so, the other apparently unchanged. 

 The specimen I exhibit to the academy was thus used; it is the portion iu pure water, and after 

 over thirty months of standing at rest is not yet so clear as its companion portion in salt water 

 became (during the lecture) in less than thirty minutes. 



When a solution of common salt (or of sea-water) is added to muddy water, the suspended 

 clay curdles or flocculates and immediately begins to fall, and in a comparatively short time the 

 liquid becomes clear. If the clear part be decanted, an equal volume of distilled water be added, 

 the sediment again diffused thi'ough the liquid by agitation, and the process be repeated, the 

 saltness of the solution being reduced at each dilution, the behavior of the same identical clay in 

 the same quantities of solution of different degrees of strength may be observed, if time enough 

 be given to the observations. We may say, iu a general way, that the stronger the solution the 

 quicker the precipitation; but the raj)idity is not directly as the quantity of salt dissolved. 

 Eeducing the saltness one-half does not necessarily double the time required for the solids to 

 settle. W^ith some clays the precipitation in a solution as strong as sea-water, or even half as 

 strong, is as much in thirty minutes as in as many days, or even months, if the water be i)ure; 

 and if the amount of salt be increased, the rapidity of sedimentation is not correspondingly 

 increased. On dilution by decantation as described, the i>recipitation becomes slower and slower. 

 When the liquid contains but one-tenth or one-twentieth the amount of salt found in sea-water, 

 the precipitation becomes very slow, but in a few weeks or months the liquid becomes as clear and 

 pellucid as the clearest natural waters. As the dilution goes on and the water contains less and 

 less salt, its capacity becomes greater for holding the clay iu suspension, both as to the quantity 

 that may be suspended and the length of time it will hold it. The identical mud previously thrown 

 down rapidly in salt or brackish water, when the water becomes fresh is again picked up, on 

 agitation, and is again held in suspension. This may be repeated indefinitely. Each time salt is 

 added the settUng is hastened, and with each freshening the mud is again suspended longer. 



I have tested this with numerous muddy waters produced artificiallj', and also on the actual 

 river water taken from the Mississippi River below New Orleans, and from the Missouri River, 

 2,800 miles above, and on other natural mnddy waters. Different specimens behave somewhat 

 differently as to degree and in details, but the essential facts are the same for all the samples I 



