276 The Art of Dyeing. 



The testing is now completed. The portions of cotton 

 which have been decolourized are to be counted. Should 

 they amount to 6, it follows that 6 chemical blue pieces of 

 cotton to be deprived of colour require 1 gran of good 

 potash. If another specimen of potash is now examined, 

 and it is found that 1 gran of this can decolourize only 3 

 pieces of the same cotton, it follows that the last is only 

 half as good as the first specimen. 



It is obvious, that in this method of testing, the same 

 cotton must always be employed which served at first for 

 the examination of the good potash. 



If it is wished to determine in this way the actual quantity 

 of potash in a ley, the first trial, upon which the estimate 

 in future trials depends, should always be made with 1 gran 

 (•959 gr.) of pure potash instead of the ley. 



For a correct estimate, let \ loth or 120 grans (HS'OSgrs.) 

 of potash be weighed out, and dissolved in 59 \ loth (27*9 oz.) 

 of water; \ a loth ("234 oz.) of this solution which may be 

 employed for examination contains 1 gran (*959 gr.) 



Carbonate of Soda. — Crystallized carbonate of soda pos- 

 sesses a similar action to potash. It is only used, however, 

 when the potash cannot be employed in consequence of its 

 containing foreign salts ; for example, as an addition to 

 saturate the acid of the aluminous mordant, and to clear 

 madder-red by depriving it of the trace of yellow. 



In the exhibition of the oil mordant the potash is used. 

 Carbonate of soda does not answer. 



Carbonate of soda can be rendered caustic like potash, 

 or can be deprived of its carbonic acid by means of lime. It 

 should be observed, that, in general, 143 lbs. crystallized 

 carbonate of soda and 120 lbs. of good potash bear the same 

 proportion to the lime employed. 



Carbonate of soda does not become moist in the air, 

 but effloresces into a white powder. This renders it fit for 

 printing. A case seldom occurs, however, where it is 

 proper except in a colourless ground of chemical blue, to 

 discharge the yellow pattern. The thickening is produced 

 by starch gum. 



As carbonate of soda is often mixed with glauber salt 

 which deteriorates it, it is necessary to submit it to chemi- 

 cal examination. The soda should be tested in the same 



