SULPHATE, NITRATE, OXALATES, AND CITRATES OF POTASH. 189 



5°- Sulphate of Potash. — This compound is formed by adding pearl- 

 ash to dilute sulphuric acid (oil of vitriol) as long as effervescence ap- 

 pears, and then evaporating the solution. It is a white saline sub- 

 stance, sparingly soluble in water, and has a disagreeable biuerish taste. 

 It exists in considerable quantity in wood-ash, and in the ash of nearly 

 all plants, and is one of the most abundant impurities in the common 

 potash and pearl-ash of the shops. This sulphate itself is not an article 

 of extensive manufacture, but it exists in common alum to the amount 

 of upwards of 18 per cent, of its weight. ^ 



Dissolved in 100 times its weight of water, the sulphate of potash has 

 been found to act favourably on red clover, vetches, beans, peas, &c., 

 and part of the effect of wood ashes on plants of this kind is to be attri- 

 buted to the sulphate of potash they contain. Turf ashes are also said 

 to contain this salt in variable quantity, and to this is ascribed a portion 

 of their efficacy also when applied to the land. 



6°. Nitrate of Potash, or saltpetre, is a well known saline substance, 

 of which mention has already been made in the preceding lectures. [See 

 p. 56, and pp. 159 to 163.] It contains potash and nitric acid only, and 

 may be readily formed by dissolving pearl-ash in nitric acid, and eva- 

 porating the solution. It exists, and is. continually reproduced in the 

 soil of most countries, and is well known to exercise a remarkable influ- 

 ence in accelerating and increasing the growth of plants. 



7°. Oxalates of Potash. — These salts exist in the common and wood 

 sorrels, and in most of the other more perfect plants in which oxalic 

 acid is known to exist. [See pp. 47 and 137.] The salt of sorrel is the 

 best known of these oxalates. This salt has an agreeable acid taste, 

 and is not so poisonous as the uncombined oxalic acid. 



When this salt is heated over a lamp, the oxalic acid it contains is de- 

 composed, and carbonate of potash is obtained. It is supposed that a 

 great part of the potash extracted from the ashes of wood and of the 

 stems of plants in general, in the state of carbonate, existed as an oxa- 

 late in the living tree, and was converted into carbonate during the com- 

 bustion of the woody fibre and other organic matter. This compound, 

 therefore, in all probability, performs an important part in the changes 

 which take place in the interior of plants, though its direct agency in 

 affecting their growth when applied externally to their roots has not 

 hitherto been distinctly recognized. It is probably formed occasionally 

 in farm-yard manure, and in decaying urine and night-soil, but nothing 

 very precise is yet known on this subject. 



8°. Citrates and Tartrates of Potash. — These salts exist in many 

 fruits. The citrates abound in the orange, the lemon, and the lime — 

 the tartrates in the grape. When heated over a lamp, they are decom- 

 posed, and like the oxalates leave the potash in the state of carbonate. 



In the interior of plants, both potash and soda are most frequently 

 combined with organic acids (oxalic, citric, tartaric, &;c., for an ac- 

 count of the most abundant of which see Lecture VI., p. 121,') and the 

 compounds thus formed are generally what chemists call acid salts — 

 that is to say, they generally have a distinctly sour taste, redden vege- 

 table blues, and contain much more acid than is found to exist in cer- 

 tain other well known compounds of the same acids with potash. 



The citrates and tartrates are not known to be formed in nature, ex- 



