The clay group has relatively the widest limits, which is unfortunate, as this is the most important of all the groups on account of the exceedingly small size of the particles. The figure 0.0001 millimeter is taken as the lowest limit of the diameter of the clay particles. These particles have been heretofore assumed to be ultra-microscopic, but by the use of a microscope of high power with oil-immersion objective and staining fluids, it has been possible to define the clay particles in a turbid liquid which has stood so long as to be only faintly opalescent. Pending more exact measurements, the figure 0.00255 millimeter has been used as the diameter of the average sized particle in the clay group. The following table gives the approximate number of grains per gram in the different types of subsoils calculated from the mechanical analysis of the typical soils already given: NUMBER OF PARTICLES OF EACH CLASS IN ONE GRAM OF SOIL. ─────────────────┬─────────────────┬─────────────────┬───────────────── Silt classes. │ No. 1 │ No. 2│ No. 3 Diameter (_d_) in│ Early truck. │ Truck and small│ Tobacco. centimeters. │ │ fruit.│ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 3 0.075 │ 85│ 34│ 102 0.0375 │ 5,511│ 4,011│ 1,900 0.0175 │ 37,230│ 54,610│ 11,890 0.0075 │ 207,500│ 199,700│ 267,900 0.003 │ 2,073,000│ 1,355,000│ 8,092,000 0.00075 │ 38,210,000│ 35,360,000│ 140,900,000 0.000255 │ 1,915,000,000│ 3,918,000,000│ 6,637,000,000 │ —————————————│ —————————————│ ————————————— │ 1,955,000,000│ 3,954,973,355│ 6,786,273,795 ─────────────────┼─────────────────┼─────────────────┼───────────────── Silt classes. │ No. 4 │ No. 5 │ No. 6 Diameter (_d_) in│ Wheat. │Grass and wheat. │ Limestone. centimeters. │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 12 0.075 │ 726│ 4│ 60 0.0375 │ 8,273│ 181│ 157 0.0175 │ 32,340│ 5,556│ 1,456 0.0075 │ 554,100│ 202,600│ 125,900 0.003 │ 3,962,000│ 10,670,000│ 8,231,000 0.00075 │ 73,990,000│ 154,900,000│ 199,900,000 0.000255 │ 10,150,000,000│ 14,570,000,000│ 19,430,000,000