tical Hue of thought, so to speak, without proceeding along the same dead 

 level: it is not a circle but a helix that marks out the path whereby 

 science has led its students. It is of importance to recollect this, for, un- 

 less we do so, the lay mind may be misled into thinking that, because in 

 some respects Liebig was wrong, Van Helmont was necessarily in all 

 respects right. The direct consequence of Liebig's mineral theory was the 

 view that the fertility, if not the productiveness, of any soil could be in- 

 ferred from a complete chemical analysis of that soil. This view, it is on ail 

 hands conceded, overstates the facts, but to conclude, from this concession, 

 that chemical analysis of soils, no matter how made and applied, is value- 

 less, would be a betrayal of ignorance in regard to the general tendency 

 of all subsequent research. 



Assuming, without argument for the present, that chemical analyses 

 of soils have some value, the way along which such analyses should pro- 

 ceed may very pertinently be discussed. But before this can even be con- 

 sidered it becomes necessary to enquire more closely into the method by 

 which nutriment is conveyed to plants. Van Helmont, as has been seen, 

 attributed this function wholly to water, a view which was vigorously con- 

 tested by Liebig, who ascribed the preparation of the food of plants from 

 the mineral constituents of the soil to the acid excretions of the roots 

 themselves. As hinted above, there is a strong inclination in some quar- 

 ters to-day to accept the substantial correctness of Van Helmont's theory, 

 at all events in an expanded form. 



A considerable recession has taken place from some of the views 

 which found currency when first Liebig's mineral theory became widely 

 accepted. Thus the complete chemical analysis of a soil has, for more 

 than a score of years, ceased to be reckoned an index of fertility. Chemists 

 of a later day, alive to the fact that the roots of plants are not able to ab- 

 sorb from the soil all the plant food constituents there present, began to- 

 modify the character of the solvents used in the laboratory for extracting 

 this nutriment: they accordingly directed their endeavours towards ex- 

 tracting from the soil only such constituents, and those only in such quan- 

 tities, as the plant rootlets are actually capable of withdrawing. The 

 proportions of plant food constituents thus removed were said to be pre- 

 sent in the soil in a form " available " to plants. Whatever the quantity 

 of plant food there may be in any soil, unless it was present in a form 

 available to the plant, it could as well, for all practical purposes, be non- 

 existent. As soon as this view began to be held, it became necessary, if 

 the analysis of a soil was to have any value for agriculturists, to employ 

 weaker solvents than those at first adopted, solvents, obviously, that 

 would simulate the action of the plants themselves. That was the proce- 

 dure which Professor Cossa urged in 1866. 



Now arose the problem of finding suitable solvents: various proposals 

 were made, and, for the purpose of putting their adaptability to the test, 

 constant and prolonged comparisons between laboratory and field experi- 

 ments were rendered necessary. A full discussion of these would be need- 

 less here; suffice it to say that the German experiment stations adopted 

 Hydrochloric acid diluted to a certain degree, and left in contact with a 

 specified weight of soil, for a definite period of time, at a fixed tempera- 

 ture. At Halle a method of determining a-vailable phosphoric oxide in 

 soils, by extracting it with citric acid solution, was introduced, and has 

 since become generally recognised for that purpose, the whole scientific 

 world over. An extension of this method was proposed by Dr. Bernard 

 Dyer in 1894, and is now usually associated with his name. Of late years 

 American investigators have begun to employ very small proportions of 

 pure water, basing their practice on the view that the soil water is the 



