8 BULLETIlSr 1122, U. S. DEPARTMENT OF AGRICULTURE. 



persing some of the colloidal material, reliable data on the non- 

 colloidal absorption in a soil can not well be obtained from an investi- 

 gation of soil fractions. It was necessary, therefore, to attack the 

 problem in another manner. 



If the noncoUoidal part of the soil is made up of various mineral 

 particles above the arbitrary limit of 1 micron in diameter, a deter- 

 mination of the absorptive capacities of soil minerals in the proper 

 state of subdivision should afford data for calculating the absorption 

 of the noncoUoidal soil constituents. Data obtained in this way 

 should be more reliable than that yielded by the preceding method 

 of investigation, as mineral powders can be prepared which are 

 essentially free from colloidal material. 



Some 21 minerals are of common occurrence in soils. Although 

 only a few of these are present in a soil in any considerable quantity, 

 it was thought advisable to determine the absorptive capacity of 

 each of them. Samples of the 21 minerals were kindly provided by 

 the National Museum. They were characteristic mineral specimens, 

 and were apparently free from decomposition products, although 

 not absolutely pure, a few containing traces of calcite. 



The work of previous investigators indicates that the absorptive 

 capacity of any one mineral powder is somewhat proportional to the 

 size of the particles,^ and that some minerals have very low absorp- 

 tive capacities. It was, therefore, advisable to test samples in a fine 

 state 01 subdivision in order to obtain an accurate comparison of the 

 absorptive capacities of different minerals. The absorptive capaci- 

 ties of larger particles could then be calculated closely enough for our 

 purpose from the values of the fine particles, on the assumption that 

 the absorptive capacity of any one kind of mineral powder is to a 

 large degree inversely proportional to the size of the particles. 

 Accordingly, a sample oi each mineral was prepared which contained 

 only particles ranging in size from 1 to 50 microns in diameter. This 

 group of particles is just above the size we arbitrarily set for the col- 

 loids and corresponds to the ^'fine fraction" separated from the soils 

 in the study just described. 



PREPARATION OF MINERAL POWDERS. 



After the minerals had been criished to small pieces in an arbor press 

 and in a steel mortar, they were ground either in an agate mortar or 

 in a steel ball mill, depending on the hardness of the mineral. The 

 harder minerals were more effectively handled in the baU mill. The 

 samples were frequently passed through a 130-mesh sieve in order to 

 obtain the desired fineness with a minimum amount of material finer 

 than 1 micron. The sieved powders were then further graded by 

 subsidence in absolute alcohol in order to remove particles that were 

 larger than 50 microns and smaller than 1 micron in diameter. Abso- 

 lute alcohol was used for the subsidence rather than water, in order to 

 prevent the formation of colloidal matter by hydrolysis of the mineral. 



The mineral powders were carefully examined under the micro- 

 scope and counts made of the approximate number of particles present 

 in the three following groups: 0.025 to 0.050 millimeter, 0.002 to 

 0.025 milluneter, and under 0.002 millimeter. There were but 



7 The data of Ammon (/) and o fVon Dobeneck (8) show that while the absorption ofany one mineral 

 increases with the degree of subdivision, the increase in absorption is somewhat less than the increase 

 in surface exposed. 



