- The velocity of propagation of temperature waves will be the same under similar conditions in two substances which possess the same
**diffusivity**, although they may differ in conductivity. - No calorimetric observations are required, but the results are obtained in terms of the thermal capacity of unit volume c, and the measurements give the
**diffusivity**klc, instead of the calorimetric conductivity k. - The method of deducing the
**diffusivity**from these curves is as follows: - The total quantity of heat absorbed by the soil per unit area of surface between any two dates, and any two depths, x' and x", is equal to c times the area included between the corresponding curves. - We thus obtain the simple equation k'(de'/dx') - k"(de"/dx") =c (area between curves)/(T - T'), (4) by means of which the average value of the
**diffusivity**klc can be found for any convenient interval of time, at different seasons of the year, in different states of the soil. - For the particular soil in question it was found that the
**diffusivity**varied enormously with the degree of moisture, falling as low as ooio C.G.S.

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