3i6 NATURAL SCIENCE. May. 



volumes for the reliability of the data. In certain other cases, 

 however, there is a wide and unfortunate divergence, mainly among 

 the clovers ; but since, in these cases, one duplicate was grown in a 

 pot under the unhealthy conditions of a London roof, the small 

 amount of acidity found is readily explained. We will not linger 

 over these interesting tables, only drawing attention to the high 

 acidity found in A rmevia, CEnothem, and pre-eminently in the Rosaceae 

 — the last observation being one that may possibly be found hereafter 

 to have important bearings upon fruit-culture. 



Having obtained this mass of data. Dr. Dyer's next trouble was 

 to decide what would be a fair general average to take as the strength 

 of citric acid solution for his soil-extractions ; and finally he decided 

 to adopt Stutzer's original proposal of a i per cent, solution as a fair 

 approximation to natural conditions. It was objected, in the dis- 

 cussion which followed the reading of his paper, that this was too 

 strong a solution, since, according to his tables, the average acidity 

 of the chief agricultural plants, viz., Cruciferae, Leguminosae, and 

 Graminaceae, was about ^ per cent. — that of the Graminaceae being 

 almost without exception below i per cent. There was much apparent 

 reason in this objection ; but it was almost met by experiments that 

 Dr. Dyer had made on the solvent powers of various strengths of 

 solution on the soils : the following figures, showing the percentage of 

 phosphoric acid dissolved from the soil by citric acid solutions of 

 various strengths, will illustrate our meaning : — 



■125 per cent, citric acid solution dissolved -0200 P.2 O5 

 •25 



5 >i It I > 



I'o ,. 



50 



It is obvious that the difference between the solvent powers of a 

 ^5 and a I'D per cent, solution is small ; and this fact much diminishes 

 the weight of the objection. Moreover, any agricultural analyst, with 

 Dr. Dyer's data before him, could adapt his solutions so as to 

 •correspond exactly with the acidity of the crop which it was proposed 

 to grow on any particular soil. 



We must now proceed very briefly to indicate the happy 

 verification of these conclusions that Dr. Dyer was able to obtain. 

 Everybody has heard of the Rothamstead experiments, and knows 

 "that for over fifty years various plots of land there have been treated 

 in every imaginable way as regards manures and crops, and complete 

 histories of every plot preserved. Samples of these soils were placed 

 at Dr. Dyer's disposal by Sir J. Lawes ; and their analytical 

 examination by the new method conclusively showed that the amount 

 of phosphoric acid dissolved by a i per cent, citric acid solution was a 

 fair index to the amount of available phosphoric acid and potash con- 

 tained in the soil. It is somewhat difficult to exhibit in its cogency the 

 ■evidence for this conclusion without going far beyond the limits of our 



