72 REPORT 1851. 



There is a not very distinct idea common enough with regard to water, 

 that it decomposes or becomes putrid. Now, properly speaking, this cannot 

 be ; it is entirely unchanged in all situations ; but as a vehicle for impurity, it 

 may be said in popular language to become putrid. Water does not become 

 putrid therefore in any sense but this, that the matter which it has in solu- 

 tion becomes decomposed ; this putrefiable matter is obtained by the passage 

 of the water over the soil or out of the soil at a depth not sufficiently great 

 for its removal. Sir Humphry Davy says, " Common river-water generally 

 contains a certain portion of organizable matter which is much greater after 

 rains than at other times, and which exists in largest quantity when the stream 

 rises in a cultivated country." A good deal of this matter is in very fine 

 suspension, and without making it appear very turbid takes away at least from 

 the brilliancy of the water. To give instances will be unnecessary ; water 

 taken from the surface and allowed to stand, and water from a deep well, or 

 water taken from a drain which runs clear, may be compared easily by any 

 one. The soil contains an abundance of organic matter, but the under soil 

 contains little or none, diminishing as v.e go down. The line of demarcation 

 between tlie organic and inorganic portion of the soil is very distinct ; if 

 water be filtered through the upper soil even for a great length of time, the 

 organic matter is not removed from it; it will even be dissolved out, whereas 

 the soil immediately below it has first very little and then no organic matter 

 in it, whilst the constant draining downwards does not drag down to it the 

 soil of the surface ; the two remain distinct portions of the organism, so to 

 speak, of the soil. The use of this is also sufficiently clear; in growing, the 

 plants require that the food should be in solution, and the water dissolves it ; 

 but when it passes away tlie plants require that it should leave the food be- 

 hind it, and accordingly it leaves it. 



Although the soil is acid, it is worthy of remark that waters flowing 

 from soils, river-waters and well-waters are alkaline, made so by lime- 

 salts generally, and also by niagnesian and alkaline salts. It is remarkable 

 with what rapidity the lime is dissolved, and how steadily the hardness of the 

 water is preserved at one point for years. The acidity of the soil must first 

 be neutralized by the lime, and an additional quantity be then taken up, the 

 lime compound being retained in the filtering medium, and the organic mat- 

 ter thereby prevented from removal. But the power of the soil to retain 

 matter does not depend on any mere formation of insoluble compounds, on 

 alkalinity and acidity ; but entirely on the action as a filter on what may 

 perhaps be termed its mechanical power, although it is not purely so. 



To show this, a solution of peaty matter was made in ammonia; the solu- 

 tion was very dark, so that some colour Avas perceived through a film only the 

 twentieth of an inch in thickness. This was filtered through sand, and came 

 through perfectly clear and colourless, having still a great excess of ammonia. 



Acetic acid was added to it until it became acid ; this solution was perfectly 

 clear (I may mention that acetic acid does not dissolve it if used first) ; it 

 passed acid through the filter of sand, and became colourless. 



Thinking to try the power of such a filter, I dissolved some organic matter 

 in strong sulphuric acid, to see if the strength of the acid would not act as a 

 counter agent to the great decomposing power of the sand. The acid was 

 allowed to pass through almost pure, although the depth was only about four 

 inches. 



The same was tried with muriatic acid, which was black by the mixture of 

 some organic substance ; it passed through the sand rapidly, and was in fact 

 brilliant. 



Sometimes, if the purifying is not completed by merely passing through 



