OF CLAY SUSPENDED IN WATER. 131 



those of ammonium chloride and nitrate ; and yet mud settles much 

 more slowly in water than in any of the solutions. 



Clay does not deposit so quickly in acid solutions as in solutions 

 of salts. The order of settling for the four acids hydrochloric, 

 nitric, sulphuric, and acetic acids of 16 per cent, was 



1. Nitric acid. 



2. Hydrochloric acid. 



3. Acetic acid. 



4. Sulphuric acid. 



The difference between the rates in hydrochloric and acetic acids 

 was slight, but still perceptible. Hydrochloric acid has a solvent 

 action on the iron contained in the mud. 



Mud deposits more rapidly in a solution of caustic soda than in a 

 solution of potash ; and as potassium hydrate evolves more heat on 

 solution than sodium hydrate, the rule appears to hold in their 

 case. 



It appeared desirable to ascertain if the rate of deposition varied 

 with the density of the solution. For this purpose four solutions of 

 common salt were prepared, the strongest containing 100 grains 

 dissolved in 500 cub. centims. of water; the second was half- 

 strength, the third one quarter, and tho fourth one eighth. 



The specific gravities of the solutions were : — 



Common salt. sp. grav. 



100 grams dissolved in 500 cub. centims. water. .1-1222 



50 

 25 

 12-5 



1-0631 

 1-0220 

 1-0122 



The mud deposited in the order of their specific gravities; 

 it precipitated most quickly from the strongest solution. It thus 

 appears, that, although probably the specific gravity of the solution 

 has some influence on the rate of settling, the real cause is most 

 probably the absorption of heat ; for the more salt passes into solu- 

 tion, the more heat is absorbed. This explanation is confirmed by 

 the deportment of fine mud in water of different temperatures. It 

 is well known that precipitates will settle more rapidly if the liquid in 

 which they are suspended be boiled. This phenomenon is, I believe, 

 usually attributed to the coagulation of the precipitate ; and in some 

 cases, for instance gelatinous precipitates, this undoubtedly happens ; 

 but in the case of finely divided precipitates I am disposed to attri- 

 bute the settling to the absorption of heat by the water. 



Experiments were made with a view to decide this point. The 

 water, however, after being heated up to the required temperature, 

 must be kept at that temperature by means of a heat equal on all 

 sides, while the mud is depositing. This was done by means of 

 water-baths, in which the beakers containing the water and mud 

 were placed. In every case it was observed that the hotter the 

 water the more rapidly the mud settled. 



Perhaps I may be allowed to attempt a possible explanation of 



