428 On Dry Leaven, 



the stock is exhausted it is very troublesome to re- 

 commence the operation. 



A substitute is found in barm or yeast. This is the 

 froth or scum that rises during the fermentation of malt- 

 liquor. It consists of a portion of the farinaceous 

 matter of the malt, associated with an abundance of 

 fixed air. This latter answers the purpose of impart- 

 ing to dough, an approved degree of lightness ; but 

 can be procured by none but those who dwell in the 

 neighbourhood of breweries, and that only during the 

 cooler season of the year while the vats are working. 



Another method of raising the dough is by the ad- 

 dition of carbonated potash. A small quantity of this 

 alkaline ley, mingled with the paste, assists in the en- 

 largement of its volume. It is employed by the house-^ 

 wives of Long-Island, in preparing an extemporane- 

 ous cake, C'dWed potashed cake, which, when properly 

 managed, is palatable and excellent. The heat of the 

 oven, probably separates carbonic acid gas enough for 

 this effect. But the quantity is insufficient to make 

 bread of the first rate. Its agency therefore is much 

 limited. 



Of those three materials, the two former are subject 

 to the inconvenience of souring, spoiling, running out, 

 or failing. This is owing principally to their moisture. 

 In leaven, a fermentation must be excited for the pur- 

 pose of producing fixed air ; while in yeast the air is 

 the product of a previous fermentation. It has there- 

 fore, for a long time been the object of particular de- 

 sire, to possess a dry composition that would readily 

 yield a sufficiency of gas for preparing good bread. 



Neither potash nor pearlash corresponded fully with 



