'216 DR. DAPBENY ON THE ROTATION OF CROPS, ETC. 



as having been detected ; but, without presuming to bring forward the analyses made 

 in my laboratory as in themselves sufficient to justify the public in rejecting the 

 former as inaccurate, I may be permitted to observe, that it is much more easy to 

 conceive that the amount of soda present may have been overrated, than that it should 

 have been estimated below its real amount, supposing anything like an equality of 

 skill and attention on the part of the operator. 



To overrate it, we need only attribute to him some degree of negligence, either in 

 not converting by means of chloride of platinum the whole amount of chloride of 

 potassium into the sparingly soluble double chloride, or in not determining its entire 

 quantity ; to estimate it too low, we must imagine, what is far less probable, a portion, 

 of the readily soluble compound of chlorine with sodium, or the equally soluble 

 double salt which the latter forms with platinum, to remain attached to the chloride 

 of potassium and platinum, and thus to add to its weight. 



Our results may also appear to militate against the conclusions of a much higher 

 authority than Sprengel, I mean Professor Liebig, who has lately represented that 

 one alkali may be substituted for another in the organization of a plant, and that a 

 species, which in inland spots assimilates a certain amount of potass, takes into its 

 fmme an equivalent proportion of soda in maritime districts, where the latter alkali 

 abounds. 



With the slender data before me, it would be the height of presumption to impugn 

 the generalisations of this distinguished philosopher, but it will be seen from the 

 analyses given below, that no difference in the nature of the alkaline ingredients 

 could be detected between barley, taken from the neighbourhood of the sea, whether 

 from the eastern or western coasts of this country, and from the more central region 

 of Oxfordshire. 



Two ingredients mentioned by Sprengel as existing in the ashes of plants were 

 searched for in a few of those to which this paper refers, but without success. These 

 were alumina and manganese, the former so universally present in the soil, tlmt it 

 may readily find admission into the ashes of the plants, unless the greatest care be 

 taken to clean off every particle of dirt entangled by their i*oots ; the latter, as 

 Liebig thinks, an accidental ingredient, being taken up by many plants in consi- 

 derable quantities where the soil contains much of it, but altogether wanting in 

 the same vegetables cultivated elsewhere. 



In order to ascertain the presence of alumina, the ash Wcis dissolved in muriatic 

 acid, the solution evaporated to dryness, in order to separate the silica, and then re- 

 dissolved in muriatic acid diluted with water. 



An excess of ammonia was afterwards added to the filtered liquor, and the preci- 

 pitate which fell, after having been well-washed, was boiled with a pure solution of 

 potass. The portion dissolved was then filtered, neutralized with muriatic acid, and 

 treated a second time with ammonia. If any precipitate had been thrown down, the 

 presumption would have been that it consisted of alumina, and the appropriate tests 

 would have been applied to confirm the conjecture ; but in the only instance in which 



