90 MICHIGAN ACADEMY OF SCIENCE. 



in actual practice caimot be Icuown, but couversely the pore space may 

 readily be determined and the angle determined from it. 

 . The length of pore in the soil is greater than the length, or height, 

 of the soil column through which the air is aspirated and depends 

 upon the angle as well as the height of the soil column. The cross 

 section of the pore has for one of its functions the angle as well as the 

 size of the grain, and conversely the size of the grain will depend upon 

 the cross section of the pore. The rate at which air may be asi>irated 

 through a soil will depend upon the size and length and number of its 

 pores, upon the pressure under which it is forced through the pores, 

 and upon the viscosity of the air which in turn depends upon the tem- 

 perature of the air. The length of pore will depend, as before stated, 

 upon the height of the soil column. The number of pores will depend 

 upon the cross section of the soil column, whence we have derived 

 the formula: 



d^=rK^^ [8.9431-10] 



Where d is the diameter of soil grain, 



K a factor dependent upon the per cent of pore space, 



h height of soil column, 



s cross section of soil column, 



p pressure in c. m. of water, 



t the time in seconds required to asi)irate 5 liters of air at 20° 

 centigrade, and [8.9431-10] is the logarithm of a constant. 



Such a soil, however, as Prof. Slichter hypothecates is never found 

 and probably seldom approached in nature. It is probably approached 

 more nearly in form than in uniformity of size. But whatever the 

 irregularities in a given soil, it is found, as has been partly stated 

 before, that its minimum pore space and its power to allow the passage of 

 a fluid through its pores are practically constant, and, as Prof. Slichter 

 says: ''It would probably be admitted that no matter how complex 

 a soil may be there exists a certain ideal soil of uniform spherical 

 grains that will transmit, under given conditions, the same amount of" 

 fluid (he says water) ''that would be transmitted by the complex soil. 

 The size of the grain of this ideal soil of the same transmission capacity 

 as the com])lpx soil we shall call the eft'ective size of grain of the 

 complex soil."" This is the term applied to the size of grain as deter- 

 mined bv the new method. 



Xni: MODE OF OPERATION. 



To perform an analysis the soil is prepared by drying and pulver- 

 izing in a mortar, using a rubber pestle. It is then sifted through a 

 1 mm. mesh sieve to remove gravel. 



It is th«m introduced into the tube (a), llie end of which is provided 

 with a light fltting cap, and the tube held firmly iijxm some solid surface, 

 and lightly tapped with a light mallet or stick. As the soil settles 

 more soil is added, and this is continued until no further settling occurs. 

 The sur|)lus soil is stricken ott", and that which I'cmnins is smoothed 

 down with some plane surface. In Prof. King's laboratory a piece of 

 ground glass is used for the purpose. The tube and contents are now 

 weighed. Knowing the volume and weight of the tube, and the specific 

 gravity of the soil, it is an easy matter to determine the pore space 



