Essay on the Analysis of Soils. 



51 



of the instrument. This last is the principal object of analysis^ 

 and that to which Sir Humphrey Davy usually confined his atten- 

 tion^ merely noticing- the proportion of coarser sand in the soil. 

 It contains^ no doubt, the great principle of fertility and nutrition ; 

 and the effect of the coarser parts may be considered as chiefly 

 mechanical. But they may much affect the fertility of the finer 

 parts, and are of the greatest importance to the soil in which they 

 are blended : they consequently deserve a more minute examina- 

 tion, to which we will return. In the mean time our attention 

 shall be directed to the composition of the finer earth in No. 4, 

 which is mixed with water in a semi-fluid state. This is well 

 shaken, and suddenly poured into a deep glass vessel, and allowed 

 to settle for a few minutes, when the heavier earth, which is sand, 

 will be deposited, and the lighter may be poured off suspended in 

 the water. It requires some little practice to effect this at once, 

 but a few trials will soon enable any one to do it. This operation 

 may be repeated until all the sand, of which the particles are 

 visible to the naked eye, is separated. The earth and water de- 

 canted out of this last vessel are now poured into a glass tube, 

 18 inches long (No. 1), the bore of which is less than an inch : 

 one end is stopped with a cork fitted into it, and the other has a 

 small lip for the convenience of pouring out the contents. In a 

 short time there will be a further deposition of earth, which will 

 be principally alumina. What remains suspended in the water 

 over it is gently poured off into another similar tube (No. 2) : this 

 will contain nearly the whole of the humus, which will take some 

 hours to be deposited in the form of a fine brown mud. The con- 

 tents of the tube No, 1 may now have a little more water added 

 to them ; after being well shaken, the tube may be set upright, 

 and left for half an hour to settle : what remains suspended in the 

 water after this must be added to the humus in the tube No. 2. 

 After some time this will also be deposited ; and the clear water 

 may be decanted off. The mud which remains is put on filtering- 

 paper in a glass funnel, and, when all the water has drained from 

 it, it is dried over the fire, and weighed. This is the most im- 

 portant portion of the soil.'^ The fine earths deposited in the 



* The dark mud which is last deposited, and which we call humus, for 

 :Want of a better name, contains no doubt a considerable portion of ex- 

 tremely fine earth, which may be detected by heating- it red-hot in a cru- 

 cible, until all the carbonaceous matter is burnt and converted into 

 carbonic acid gas. This may be accelerated by throwing into the crucible 

 small portions of nitrate of ammonia. The oxygen of the nitric acid will 

 unite with the carbon, and the nitrogen and ammonia lly off in the form of 

 gas, leaving no residuum but the mineral earths and salts. But the organic 

 matter being destroyed by this operation, its quantity alone can be discovered. 

 The vegetable matter, which gives the soil its fertility, all other circum- 



