528 



SCIENCE 



[N. S. Vol. XLII. No. 1085 



acids of different strengths, and for different 

 times. The results of this investigation were 

 published in 1873, in the American Journal of 

 Science, and in the Proceedings of the Amer- 

 ican Association for the Advancement of Sci- 

 ence, having been read before that association. 

 It was conclusively shown that there was a 

 steady increase in the extraction of potassium 

 for five days, remaining stationary afterwards, 

 the amount extracted during the first twenty- 

 four hours being about one half of the final 

 figure, while phosphorus, lime and magnesia 

 were fully extracted. 



Notwithstanding this demonstration, fully 

 published in two standard publications, a num- 

 ber of years later the " Official Chemists," in a 

 meeting at Washington, hastily adopted, 

 against my protest, the arbitrary ten-hours 

 digestion proposed by Kedzie, as the official 

 method to be used in state and government 

 work. 



It is no wonder that as a result of this irra- 

 tional practise, chemical soil analysis became 

 more and more discredited as a means of ascer- 

 taining the quality and permanent productive- 

 ness of soils. In cases where potassium was 

 in abundant supply, it gave results correspond- 

 ing to the field tests because of the complete 

 extraction of phosphates, lime and magnesia 

 during the ten hours' digestion. On the other 

 hand, where potassium was deficient, no definite 

 relation between the analysis and practise could 

 appear. 



But when Hopkins goes so far as to deter- 

 mine the potassium content by the fusion 

 method, thus decomposing all the resistant sili- 

 cates, feldspar-sand, etc., as well as the easily 

 decomposable zeolitic minerals, he goes far be- 

 yond the limits within which any definite cor- 

 relation between soil composition and vegeta- 

 tive action is to be expected; and whatever con- 

 clusions are based upon such analyses are prac- 

 tically groundless. Knowing as we do that 

 the assimilation of inorganic substances from 

 the soil by plants is mediated by acid solvents, 

 whether derived from the air, from vegetable 

 decay, from secretion by plant roots or bac- 

 teria, it certainly is most rational to ascertain 

 how far acid action can go in the soils under 



examination. This limit, and no arbitrary 

 rule of time, or ultimate analysis, must serve 

 as the basis of judgment for practical compari- 

 son of soil values, or producing capacity. 

 Hopkins's own experiments on the growth of 

 plants in the undissolved residue from the 

 "official" analysis simply corroborate what 

 had been abundantly shown by Loughridge's 

 work in 1873, but prove nothing against the 

 practical value of soil analyses properly made. 

 They do throw discredit upon the "-official 

 method," so far as potassium is concerned. 



But soil chemists would feel additionally in- 

 debted to Hopkins if he would undertake to 

 supplement the somewhat gratuitous proof he 

 has given of the inadequacy of the official 

 method, by growing plants on the residue 

 from a digestion carried to the limit of acid- 

 solubility ; which in the case of the soil selected 

 by Loughridge and myself we found to be five 

 days for acid of the accepted sp. g. of 1.115. 

 I have long desired to make this crucial test, 

 but have not been able to find the time or means 

 to do so. If an Illinois soil can thus be made 

 to yield to any plant a practically important 

 amount of potassium, it will be very desirable 

 to know it and thus put an end to farther 

 controversy in the matter; while rendering an 

 important service to soil investigation and 

 plant physiology. 



E. W. HiLGARD 



Univeksity op California, 

 September 10, 1915 



ELEMENTAEY MECHANICS 



To THE Editor of Science: There have ap- 

 peared in your pages recently a number of con- 

 tributions by various authors to the discussion 

 of the dynamical equation ma = f or some of 

 its possible variants. It seems as though it 

 would be necessary, for a complete discussion 

 of the relative merits of the different ways of 

 introducing a student to the dynamical equa- 

 tion cited, to enter at least briefiy upon the 

 matter of the student's previous training in 

 mechanics. We are all aware that it is at pres- 

 ent somewhat stylish to begin the study of me- 

 chanics with kinetics and to treat statics as 

 a special case in which the accelerations are 



