October 20, 1905,] 



SCIENCE. 



495 



The Report of the Manchester Museum for 

 1904^1906 notes a deficiency in the finances of 

 about $1,000, but causes one to wonder how 

 so much good work as is accomplished by this 

 institution can be done on an income of less 

 than $15,000. 



DISCUSSION AND CORRESPONDENCE. 



CONTRIBUTIONS TO OUR KNOWLEDGE OF THE 

 AERATION OF SOILS. 



Under the above title Dr. Edgar Bucking- 

 ham presents the results of a series of investi- 

 gations relating to an important subject, in 

 Bulletin No. 25 of the Bureau of Soils. As 

 a practical problem in soil management the 

 securing of those conditions which will insure 

 a deep and ample ventilation is extremely 

 needful; and hence any essential advance in 

 our knowledge of the principles governing 

 soil aeration is important. In the letter of 

 submittal it is stated: 



This paper presents for the first time definite 

 information regarding the rate at which a gas 

 escapes by diffusion from the soil into the atmos- 

 phere, or vice versa. It shows that the rate of 

 diffusion varies approximately as the square of 

 the porosity of the soil, and that this diffusion 

 follows the laws for the free diffusion of gases. 

 It thus becomes possible to calculate the rate of 

 aeration in any particular soil from results ob- 

 tained in experiments on free diffusion. Tables 

 are given showing the rate of escape (and conse- 

 quently, for a condition of equilibrium, the rate 

 of formation as Avell) of carbon dioxide in the 

 soil when thfe porosity of the soil and the con- 

 centration of the carbon dioxide at any given 

 depth are known. The paper shows further that 

 the aeration of soils is almost entirely due to 

 diffusion phenomena changes in barometric pres- 

 sure liaA'ing very little influence in comparison. 



The author in his ' Concluding Remarks ' 

 says: 



1. We have measured the rate of flow of air 

 under pressure by transpiration and of air and 

 carbonic acid by diffusion, through four widely 

 different soils, in varying states of structure, 

 compactness and moisture content. 



2. We have shown that the speed of diffusion 

 of air and carbonic acid through these soils was 

 not greatly dependent upon texture and structure, 

 but was determined in the main by the porosity 

 of the soil. 



3. We have shown that the rate of diffusion 

 was approximately proportional to the square of 

 the porosity. 



4. We have shown that Avhen this relation is 

 used to compute from our results the rate of 

 free diffusion when no soil is present, it gives a 

 result which is entirely consistent with what is 

 already known from the work of other experi- 

 menters on the free diffusion of gases. 



5. We have shown that when the porosity of a 

 soil is reduced by compacting it, the ease with 

 which air flows through it under the driving in- 

 fluence of a difference of pressure is greatly re- 

 duced, varying as the sixth or seventh power of 

 the porosity. 



6. We have investigated the depths to which 

 free outside air might penetrate soils to different 

 depths, under such barometric variations as are 

 to be expected in average cases, if the outside air 

 remained distinct from the soil air. 



7. We have shown how to compute the rate of 

 escape of carbonic acid from the soil by diffusion 

 under given conditions of pressure, temperature, 

 porosity and concentration of carbonic acid. 



8. We have compared the linear velocities of 

 diffusion and barometric transpiration, and 

 hence— 



9. We have shown that the escape of carbonic 

 acid from the soil and its replacement by oxygen 

 take place by diffusion and are determined by the 

 conditions which affect diffusion, and are sensibly 

 independent of the variations of the outside 

 barometric pressure. 



The foregoing remarks and conclusions are 

 based on the mathematical treatment of a very 

 limited series of laboratory experiments, which, 

 however, have been executed with great care. 

 The subject is one so complex and intricate 

 that it can not be solved by so short and direct 

 a cut and it is a matter for exceeding regret 

 that this piece of work, admirable in itself so 

 far as it goes, should be given out by the De- 

 partment of Agriculture with so much of as- 

 surance of finality for its conclusions before 

 they have been checked by even a single field 

 observation ot experiment. Almost infinite 

 injury is done to the cause of agricultural 

 science and to the growth of the Department 

 of Agriculture along sound and enduring 

 lines by prematurely exploiting results of in- 

 vestigation, striving to get them before the 

 public eye of practical men — congressmen, 

 farmers, merchants and manufacturers — but 



