VALUE OF DETERMINATION OF FINENESS OF GRAIN.
As before stated, fineness of grain is the probable cause of several of the other properties exhibited by clays. Since fineness of grain is the cause, and the other properties, in a large sense, the effects, the true significance of this determination can be best discussed by dealing sep arately with the properties induced by size of grain.
Numerical Results.—In Table VII is given the per tentage by weight of calcined materials in each of the several groups according to sizes of particles.
The distribution of "combined water" over the several groups, as given in Table I, was necessarily calculated by proportion, for the total loss on ignition of the finer groups in some cases amounted to two or three times that which occurred on ignition of the whole sample. Satisfactory explanation of this increase or gain in volatile matter during the process of analysis cannot be given. It is supposed, however, that it is due in part to some organic growth developed in the water, or, possibly, oil from the compressed air that was used in the siphoning off of the supernatant liquid. That this last suggestion will not account for all of this increase, if any, in the volatile matter accumulated in process of analysis, was proved by the fact that when precautions were taken to clear the air of all possible traces of solid material, there was still nearly the same increase. There is therefore considerable doubt as to the value of re-dis
tribution of the loss on ignition by means of proportions, yet the data obtained in this way are considerably more accurate than they would otherwise be.
The irregularities in the data are pointed out solely to call attention to a weak point in this most important determination. Mechanical analysis of clays, as has been stated before, bids fair to become a very essential test in determining the full value of a clay, and attention should be given to the elimination of this increase in volatile matter during the process of analysis. Soil physicists are experiencing the same difficulty, and yet they have learned to give considerable, in fact, a large amount of credit to the mechanical analysis of soils as a means of determining its proper ties for their purposes .