212 



SCIENCE 



[N. S. Vol. XL VI. No. 1183 



In drawing' his conclusions -witli regard to 

 the showing made by the different phosphates, 

 the writer was governed chiefly by a consid- 

 eration of the soil conditions and results of 

 the individual series and, as he thinks, very 

 naturally placed acid phosphate first, bone 

 meal second, and rock phosphate third in 

 profitableness. 



With all the individual series in view, let 

 us see the kind of formula Dr. Hopkins must 

 use in order to arrive at his conclusion with 

 regard to the relative standing made by bone 

 meal and rock phosphate. The formula and 

 his conclusions may be stated as follows: 



Disregard series 4, omit one half the bone- 

 meal data of series 3, include series 1 (con- 

 ducted on a soil not poor in phosphate), and 

 with the acid of series 2 obtain averages which 

 show that, as used, the bone meal returned 

 more profit than the rock phosphate. I^ow, 

 make the unwarranted assumption that the 

 profit from bone meal would decrease in di- 

 rect proportion to the quantity used, and ob- 

 tain the result that a dollar invested in rock 

 phosphate made a profit of 39 cents more than 

 a dollar invested in bone meal, or, the rock 

 phosphate was superior to the bone meal. 

 Q. E. D. 



In Science^ March 2, 1917, page 214, Dr. 

 Hopkins says : " The calculated profits men- 

 tioned in Professor Mooers's Science article^ 

 are evidently based upon different valuations 

 than those reported in the bulletin." The 

 writer finds that the calculated profits for 

 both acid phosphate and rock phosphate, as 

 given in the Science article referred to, 

 should be divided by 2. This, of course, does 

 not affect the relative standing of the two 

 materials. One dollar invested in acid phos- 

 phate shows an average profit of $2.14 per 

 acre where the cowpea crops were turned un- 

 der, and of $2.71 where removed, but one 

 dollar invested in rock phosphate gave an av- 

 erage return of only $1.29 under either con- 

 dition. The writer has assumed that Dr. Hop- 

 kins could give a simple explanation for his 

 conflicting estimates, as given in Science, 

 ZSTovember 3, 1916, p. 652, and in Science, 



3 Science, January 5, 1917, pp. 18 and 19. 



March 2, 1917, p. 214. In the former article 

 he says, " For each dollar invested rock phos- 

 phate paid back $2.29," but in the latter ar- 

 ticle he says, with regard to the same data, 

 " Easy computations show profits per $1.00 

 invested of . . . $1.29 from phosphate rock." 



From correspondence with dealers in rock 

 phosphate, the writer is informed that imtil 

 about six years ago the usual guarantee of 

 fineness for the rock phosphate sold to farmers 

 for fertilizer purposes was that 90 per cent, 

 would pass through a 60-mesh sieve, but that 

 the present guarantee is for 90 per cent, to 

 pass through a 100-mesh sieve. Dr. Hopkins 

 seems to have this in mind when he says, 

 "Raw rock phosphate is now procurable in 

 very much better mechanical condition than 

 when these experiments were conducted." * 

 That he was in error with regard to the rock 

 phosphate used in the experiments referred to 

 may be seen by reference to page 59 of 

 Bulletin 90, where the following statement is 

 made : " 90 per cent, was found to pass 

 through a 100-mesh sieve." 



In conclusion, the writer will add, that on 

 page 60 of Bulletin 90, the content of total 

 phosphoric acid in the rock phosphate was 

 stated to be 33.9 per cent. The usual guar- 

 antee and expectancy for this material, as 

 sold to farmers for fertilizer purposes, is a 

 little under 30 per cent. With perfect fair- 

 ness the calculations for phosphate rock used 

 in the experiments might have been placed 

 on the latter basis, and an increase of 13 per 

 cent, can be properly allowed — as was referred 

 to on page 59 of the bulletin — to the estimated 

 cost of the applications made. This change 

 would appreciably increase the unfavorable 

 showing made by the phosphate rock. 



C. A. MOOERS 



Agricultural Experiment Station, 

 Universitt of Tennessee 



a method for obtaining amceba 

 In comraon with many teachers I have found 

 it necessary, at the opening of college in the 

 fall, to provide large numbers of the indis- 

 pensable amseba. I venture to set down a 

 method which I have found successful during 



* Science, March 2, 1917, p. 214. 



