DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 215 



sent the weight of the alkaline salts originally present in the ash. Having ascertained 

 this, the dry residue is dissolved in a small quantity of water, chloride of platinum 

 added, and the whole evaporated nearly to dryness. It is then treated with dilute 

 alcohol, which takes up the double chloride of platinum and sodium*, together with 

 any excess of the reagent that may have been added. The undissolved residue is the 

 double chloride of platina and potass, from which the amount of the chloride of 

 potassium may be calculated. The difference between the weight of the latter and 

 of the whole salt gives that of the chloride of sodium proceeding from the ash. 



The carbonic acid present was best ascertained by operating on a separate portion 

 of the ash, and the common method of determining it by the loss of weight conse- 

 quent upon the addition of a stronger acid was adopted, with the precautions usually 

 taken -f-. 



The chlorine also was determined in the usual manner by nitrate of silver, a sepa- 

 rate portion of the ash being employed for that purpose. 



In the analyses given, it has been usual to consider it as in combination either 

 with sodium or with potassium. That this was the case, seemed probable from the 

 curious relation generally found to subsist between the quantity of chlorine and of 

 sodium detected, which in many instances approximated so nearly, that we were led 

 to conclude, that the chlorine in these instances merely implied a corresponding 

 amount of chloride of sodium existing in the ash. That the correspondence should 

 not have been exact, may be more readily explained, when we consider that the only 

 generally practicable mode of estimating soda is an indirect one, and therefore liable 

 to some degree of uncertainty. 



In the few instances where the amount of chlorine was more than proportionate to 

 that of the sodium, it was thought consistent with analogy to regard that portion of 

 the former which was in excess, as held in combination with the vegetable alkali, or 

 as representing an equivalent weight of chloride of potassium. 



This mode of stating the results may appear objectionable, as blending theory with 

 fact, but my reason for adopting it is, that it j»oints at an important general conclu- 

 sion, which it is hoped future inquiries will either negative or confirm, namely that 

 the base of the soda found in plants commonly enters them in a state of combination 

 with chlorine, being derived from the common salt, taken up, but not decomposed, 

 by the organs of the plant. 



Such an inference indeed cannot be adopted by those who receive the analyses 

 given by Sprengel as correct, for in many of these large quantities of soda are stated 



* This double chloride is readily decomposed, if first rubbed up with mercury, which flies off along with the 

 chlorine in the form of calomel, when heated. 



t It seems a defect in the analyses reported by Sprengel, that this ingredient is never mentioned in them ; 

 for although it may not be present as such in the crop, yet its amount in the ash probably represents that of the 

 organic acids existing in the plant previously to its being burnt, and hence the proportion which it bears in 

 different samples of the same species to the phosphoric and other mineral acids, may tend to indicate the rela- 

 tion subsisting between the amount of organic and of inorganic matter, arising from the mode of culture or 

 other circumstances. 



