EXPERIMENT STATION BULLETINS. 471 



The Relation 'betic-ecn Grain Size and Surface Film can l)e computed 

 fundamentally in the followinf]^ way under the supjwsition that the 

 •grains are spherical: The diameter of the grain be a mm. The volume 



of one grain is 3 (2)''^" g' '^ T'^^ average specific gravity of soil par- 

 ticles is not verj' far from 2. 0. The weight of one particle is therefore 

 "' I ^ mg. and one milligram contains ^^ particles. The surface of 

 one such particle is 4 /'Ay'r=^* ^ square millimeters and the surface of all 



particles in 1 mg. of soil is 2 "^\f~ " 26a square millimeters. In 100 g. 



of soil, the surface of all particles is ^-^^ = -^^^^ square millimeters. 



= -?^ square centimeters. 

 If b cc. of liquid are added to 100 g. of this soil, it will be spread 

 out over a surface of -^^ square centimeters, and the liquid film will 



be f = —^ cm. thick. It is more convenient to compute the surface 

 film in microns. The thickness of the moisture film with a grain size 

 of a mm. diameter and with b parts of moisture in 100 g. of dry soil 

 (not per cents of moisture) is f = 4.3 a b microns. 



This formula is accurate only with round particles. If they are 

 cube-shaped, the surface is 38% larger, and the film wall be consequently 

 38% thinner. The more irregular the surface, the thinner will be the 

 film diameter at the same grain diameter. It is hardly imaginable, 

 however, that the shape of the soil particles will be so odd-shaped as 

 to have a surface more than twice as large as that of spheres. The fac- 

 tor 4.3 may therefore become as small as 2.0 in some cases, but probably 

 not smaller. 



It may be unnecessary to call attention to the fact that a moisture 

 film as it is computed here, does not really exist. The water in the 

 soil forms by no means an even film over the entire inner surface of 

 the soil, but gathers at the places where the soil particles touch. The 

 "thickness of the moisture film" is solely an expression for a certain 

 condition in the soil, it allows us to figure with an essential factor in 

 bacterial life that cannot be expressed precisely by one single term. 

 Though the moisture film is mentioned continuously in the future dis- 

 cussions, it should always be interpreted under the just mentioned 

 limitations. 



Table VII showed the optimal moisture content for B. mycoides in 

 medium sand to be between 5 and 10%. The data obtained with 5% 

 moisture are unreliable and do not check with each other. Generally 

 the sand with 10% moisture seems to afford a better medium, and the 

 real optimum lies between the two. According to the above-mentioned 

 formula, the moisture film would be 22 — 43 /* figuring on 1 mm. 

 grainsize and round grains. Since the grains are quite angular and 

 irregular (see Figure I) the film is probably only half as thick, approxi- 

 mately 10 — 20 fi. This film would suffice to surround bacteria and even 

 smaller protozoa entirely with water. If this film decreases in thick- 



