One of the most interesting discoveries of modern chemistry is the nature of those essences which give the various flavours to fruits. These “artificial fruit essences” have been so skilfully imitated in the laboratory of the chemist that they are extensively employed to flavour confectionery, &c. Examples of these artificially prepared fruit essences, may be seen in the case. ACIDS. Many of the organic acids resemble closely in their composition starch and sugar, and may to a certain extent act on the system in the same way. In the classification adopted in the Food Collection, they are classed under _medicinal or auxiliary food_. _Acetic Acid or Vinegar_ is obtained either from the oxidation of alcohol in fermented liquors, or from the distillation of wood. Common vinegar is obtained from the oxidation of the fermented wort of malt. A series of preparations illustrating the processes undergone in the formation of vinegar from malt has been presented to the collection by Messrs. Beaufoy and Co. _Citric Acid_ is contained in many fruits, but exists in greatest abundance and purity in the fruits of the orange, the lemon, the citron, the shaddock, the pommeloe, the lime, and others. All these fruits contain citric acid, and varying proportions of sugar. Citric acid can be separated from the juice of these plants in a crystalline form. _Tartaric Acid_ is found in the juice of the fruits of the vine tribe, more especially of the common vine. This acid gives the acidity to the fruit of the grape, and is the acid present in wines. _Malic Acid_ is contained in the fruits of the natural order Rosaceæ. It has the same general properties as the other acids, and is contained alone in apples and pears; whilst in cherries, plums, &c. it is mixed with other acids. _Oxalic Acid_ is contained in the wood sorrel, also in the common sorrel, and various species of rhubarb. Species of the latter genus are extensively cultivated in this country, and the stalks of their large leaves cut up and made into pies, puddings, &c. They are ready for use early in the spring, and are an excellent substitute for fruit in pies and tarts at that season of the year. Although oxalic acid is a _poison_ when taken in considerable quantities, as ordinarily consumed it probably acts in the same way on the system as other acids. For examples of Acids, Pickles of various kinds, &c., see Case 23. ALKALOIDS ACTING AS STIMULANTS OR SEDATIVES. The next and last group is that of Food Substances containing alkaloids, which act upon the nervous system as stimulants or sedatives. The principal examples are Tea, Coffee, Cocoa, Tobacco, Hemp, and Opium.