PRESERVATION OF FRUITS BY DRYING. 187 



when concentrated by evaporation, acts upon the copper 

 boilers in which the operation is carried on, so as to form 

 an acetate of copper : this, by producing colics, would 

 render the use of thq extract dangerous, especially at the 

 south, where the principal article of food for children is 

 the raisine. In order to obviate this serious evil, an an- 

 cient and generally followed custom is observed : as soon 

 as the must of the grapes begins to boil in the coppers, a 

 bunch of keys is thrown in, and allowed to remain till the 

 operation is completed : these keys attract the copper and 

 become covered with the precipitate thus formed, and 

 nothing remains in the extract but the acetate of iron, 

 which is not injurious. 



I have observed that the juices of all succulent fruits 

 might be converted into extracts, and thus preserved for 

 use in the course of the year; but the greater part of 

 these juices, when concentrated by evaporation, are so 

 excessively acid as to be totally unfit for food, and they 

 only form, when mixed with water, a very sour drink. In 

 order to correct or conceal this acidity, these juices are 

 boiled with an equal weight of sugar, and thus made into 

 sirups and jellies. 



As it is of importance to be able to extract and preserve 

 for domestic purposes, for pharmacy, and for the arts, cer- 

 tain vegetable products, which can be only very imper- 

 fectly obtained by mechanical pressure, recourse is had to 

 other means; those liquids are made use of which will 

 dissolve the wished for principles, and the solution is 

 afterwards evaporated to dryness. 



The fluid most generally employed for solutions is wa- 

 ter; this dissolves the extractive matter, mucilage, sugar, 

 and the greater part of the salts, and mixes with the 

 mealy portions of plants ; it may be applied cold or hot to 

 the vegetables, or they may be boiled in it, according 

 to the nature of the principle to be extracted ; water will 

 dissolve all that is soluble in them, and the extracts may be 

 obtained from the solution by evaporation. 



The resins, which are found so abundantly in some 

 vegetables, are not soluble in water, and the place of this 

 liquid must be supplied by alcohol, in which the plant must 

 be digested ; evaporation will separate the alcohol from the 

 resin which it holds in solution. In order to avoid the ac- 

 cidents that might occur from the dispersion in the at- 

 mosphere of a very inflammable vapor, the evaporation 



