152 SOIL CONDITIONS AND PLANT GROWTH 



time taken to fall through a column of water of given height. When a body 

 falls through a vacuum the time taken is independent of its size or weight, but 

 if air or any other fluid is present the case becomes more complicated and the 

 proper mathematical relationship has been found by Stokes to be v = 



, where v = velocity of the falling particle, a- its density, a its radius 



(assuming it to be a sphere), and p the density and 17 the coefficient of 

 viscosity of the medium (Trans. Camb. Phil. Soc., 1851, vol. ix., p. 8). 



The numerical values at 16 C. are : g = 981, o- = 2-5, p = i, 17 = -on, 



and the equation therefore reduces to v a 2 x 29430, or a = -^-? cm. 



The calculated and observed values are found to agree fairly well, differ- 

 ences being due to the fact that the particles are not true spheres, and to the 

 existence of convection currents produced by changes of temperature. 



The method adopted by the Agricultural Education Association (see 

 Journ. Agric. Science, 1906, i., 470) is as follows: 



1. Ten grams of the air- dry earth, which has passed a 3 -mm. sieve, are 

 weighed out into a porcelain basin and worked up with 100 c.c. of N/5 hydro- 

 chloric acid, the acid being renewed if much calcium carbonate is present. 

 After standing in contact with the acid for one hour, the whole is thrown 

 upon a dried, tared filter and washed until free of acid. The filter and its 

 contents are dried and weighed. The loss represents hygroscopic moisture 

 and material dissolved by the acid. 



2. The soil is now washed off the filter with dilute ammoniacal water on to 

 a small sieve of 100 meshes to the linear inch, the portion passing through 

 being collected in a beaker marked at 10, 8-5, and 7-5 cm. respectively from 

 the bottom. The portion which remains upon the sieve is dried and weighed. 

 It is then divided into " fine gravel " and " coarse sand " by means of a sieve 

 with round holes of i mm. diameter. The portion which does not pass this 

 sieve is the " fine gravel ". This should be dried and weighed. The differ- 

 ence gives the " coarse sand ". If required, both these fractions can also be 

 weighed after ignition. 



3. The portion which passed the sieve of 100 meshes per linear inch is 

 well worked up with a rubber pestle (made by inserting a glass rod as handle 

 into an inverted rubber stopper), and the beaker filled up to the 8-5 cm. mark 

 and allowed to stand twenty- four hours. The ammoniacal liquid which contains 

 the " clay " is then decanted off into a Winchester quart. This operation is re- 

 peated as long as any matter remains in suspension for twenty- four hours. The 

 liquid containing the " clay " is either evaporated in bulk or measured, and, after 

 being well shaken, an aliquot portion taken and evaporated. In either case 

 the dried residue consists of " clay " and " soluble humus ". After ignition 

 the residue gives the " clay,'' and the loss on ignition the " soluble humus ". 



Here minimum value of v = cm. per sec., ^and the minimum dia- 



10,000 



meter of the particles works out to *oo 1 3 mm. 



