CITRIC ACID; PYROLIGNEOUS ACID. 257 



tiful scarlet dye, produced by a preparation of tin ; and for 

 altering the hue of some colours, which are exclusively used in 

 the dyeing of silk. Besides its use in the arts, Lemon-juice is 

 very largely used in the navy, for the purpose of preventing 

 the complaint termed Scurvy ; which is very apt to be brought 

 on by the continued use of salt meat by confinement, but parti- 

 cularly by the want of fresh vegetables. During long voyages, 

 a regular allowance is made to each man, which he is required to 

 use as a medicine. This, however, has been now rendered less 

 necessary than formerly, since the art of preserving meats and 

 vegetables in a fresh state has been brought into general use. 

 Citric acid exists in many of our commonest fruits^ such as the 

 cranberry, cherry, red whortleberry, and the hip of the wild- 

 briar ; whilst in the red gooseberry, the currant, the bilberry, 

 the black cherry, the wood strawberry, and the raspberry, it is 

 mixed with an equal proportion of malic acid, which exists alone 

 in apples, pears, and other fruits. It is interesting to notice the 

 uses of the acids in these situations. It has been formerly stated, 

 that gum or starch, when acted on by a vegetable acid with a 

 moderate degree of heat, is converted into sugar ; and this is 

 exactly what takes place in fruits during ripening, which pro- 

 cess consists in the conversion of the starch of the hard unripe 

 fruit into sugar, without any diminution in the amount of acid, 

 which is sometimes indeed really increased, whilst its taste is 

 concealed by the sugar. 



403. One more vegetable acid may be mentioned; though it 

 probably does not exist as such, in the substance from which it 

 is obtained, but is formed by the heat employed to set it free. 

 This is pyroligneous acid, formerly called acid spirit of wood, 

 which is procured by subjecting wood in closed iron retorts to a 

 strong red-heat ; the vapour that is given off, partly consists of 

 this acid, mixed with tarry matter, which is separated by a 

 second distillation. This acid, which in some degree resembles 

 very strong vinegar, is used by the dyer and calico-printer ; and 

 it is also employed for making pickles and other culinary pre- 

 parations, in which an acid of great strength is required. The 

 impure acid has been found to possess, in a remarkable degree, 



