Dr Colquhoun on the Art of' baking Bread. 79 



Of the several ingredients which enter into the constitution 

 of gingerbread, Dr Colquhoun has proved that the only ones 

 which are essential for rendering it light and spongy are trea- 

 cle and carbonate of potash, and there can be no doubt that 

 their mutual action consists in the evolution of carbonic acid 

 gas. But in order to establish this point with certainty, " the 

 substitution of carbonate of soda, and of carbonate of magne- 

 sia for carbonate of potash, was tried, and it invariably turned 

 out that the bread in these cases expanded just as well in the 

 oven, as when an equivalent quantity of carbonate of potash 

 had been employed. And when, on the contrary, in place of 

 these substances, there was mixed up with the dough either 

 caustic potash or caustic magnesia, the bread never expanded 

 in the slightest degree in the process of baking, whether the 

 dough was baked when recent, or after being kept a consider- 

 able time. From this it resulted, that the presence of an alka- 

 line carbonate was clearly essential to the gasifying of the gin- 

 gerbread-dough ; and it seemed almost a necessary inference, 

 that the rising of the bread during the baking is produced by 

 carbonic acid gas, and that this gas is developed in consequence 

 of some mutual action which takes place between the treacle 

 and the alkaline carbonate. ,, 



The evolution of carbonic acid gas, under these circumstan- 

 ces, can alone be attributed to the presence of a free acid in 

 treacle. " That such an acid does, in a greater or less degree, 

 always exist in treacle, seems proved by the fact, that, of many 

 specimens which were examined in the course of the experi- 

 ments just mentioned, all possessed distinct traces of acidity, 

 and to an extent sufficient to enable them to communicate a red 

 colour to vegetable blues ; but the amount of uncombined acid 

 present in all these cases appeared to be very trifling, and it 

 was difficult to ascribe to its sole agency the production of 

 effects so striking. It cannot be doubted, however, that this 

 uncombined acid must operate to a certain extent in produ- 

 cing a decomposition of the alkaline carbonate ; and it may 

 be conjectured also, that the superiority in the expanding 

 power of old dough is occasioned either by the additional 

 acidification of a small quantity of the treacle, to which it 

 would be disposed during the keeping, by its state of mixture 



