12 On Paring and Burning. 



Lime it will be seen is the principal constituent of this ash. 

 This may account for the fact that this thistle delights to grow 

 on the calcareous formation of Gloucestershire. In addition to 

 lime, this ash contains a very large proportion of salts of potash, 

 and besides sulphuric acid, soluble silica, and several other less 

 important mineral fertilizing constituents, an amount of phos- 

 phoric acid which is by no means inconsiderable. This acid 

 is evidently united chiefly with lime as phosphate of lime. 



In preparing the ash of couch, it was found extremely difficult to 

 separate adhering clay ; notwithstanding all care the removal of 

 adhering fine soil from the couch was incomplete, as will be seen 

 by a glance at the subjoined analyses. 



Couch on burning furnished an ash, coloured slightly red by 

 oxide of iron, derived no doubt chiefly from adhering clay. 



100 parts of this couch ash were found to contain by my friend 

 and pupil Mr. Kensington, now assistant to Professor Anderson 

 of Glasgow : 



Composition of the Ash of Couch. 



Potash 10-02 



Soda 5-69 



Chloride of sodium 3-34 



Lime 5*58 



Magnesia '04 



Oxides of iron and alumina 12-40 



Phosphoric acid 9 -38 



Sulphuric acid 5*33 



Soluble silica 24-92 



Insoluble silicious matter (chiefly sand) .. 17'50 



Carbonic acid, and loss 5'80 



100-00 



These constituents probably are united in this couch ash as 

 follows : 



Carbonate of potash .. 14-10 



Potash in a state of silicate . .. '27 



Soda in a state of silicate .. . .. 5'69 



Chloride of sodium . . 3*34 



Oxides of iron and alumina . . . 12-40 



united with phosphoric acid . .. 9*38 



equal to bone-earth .. . .. (20'32), 



Sulphate of lime .. 9-OG 



Carbonate of lime .. 3*30 



Magnesia in a state of silicate . . . '04 



Soluble silica .. .. .. .. ' .. .. 24-92 



Insoluble silicious matter (sand) .. ..- 17'50 



100-00 



The ash of couch it will be observed differs in several respects 

 from that of the stemless thistle. The latter contained but little 



