106 



AGRICLLTHRAL CHEMI'STRY. 



stances, consequently, in which it is inca- 

 pable of combining with an acid. 



The products of the fermentation and 

 putrefaction of neutral vegetable and animal 

 matters are generally accompanied by sub- 

 stances of an offensive odour; but the most 

 remarkable example of the generation of a 

 true ethereal oil is seen in the fermentation 

 of the Herba centaurium minorius, a plant 

 which possesses no smell. When it is ex- 

 posed in water to a slightly elevated tempe- 

 rature it ferments, and emits an agreeable 

 penetrating odour. By the distillation of the 

 liquid, an ethereal oily substance of great 

 volatility is obtained, which excites a prick- 

 ing sensation in the eyes, and a flow of 

 tears. (Biichner.) 



The leaves of the tobacco plant present 

 the same phenomena; when fresh they pos- 

 sess very little or no smell. When they are 

 subjected to distillation with water, a weak 

 ammoniacal liquid is obtained, upon which 

 a fatty crystallizable substance swims, which 

 does not contain nitrogen, and is quite desti- 

 tute of smell. But when the same plant, 

 after being dried, is moistened with water, 

 tied together in small bundles, and placed 

 in heaps, a peculiar process of decomposi- 

 tion takes place. Fermentation commences, 

 and is accompanied by the absorption of 

 oxygen ; the leaves now become warm and 

 *<iiit the characteristic smell of prepared to- 

 bacco and snuff. When the fermentation is 

 carefully promoted and too high a heat 

 avoided, this smell increases and becomes 

 more delicate; and after the fermentation is 

 completed, an oily azotised volatile matter 

 called nicotine is found in the leaves. This 

 substance nicotine, which possesses all the 

 properties of^a base, was not present before 

 the fermentation. The different kinds of 

 tobacco are distinguished from one another, 

 like wines, by having very different odorife- 

 rous substances, which are generated along 

 with the nicotine. 



We know that most of the blossoms and 

 vegetable substances which possess a smell 

 owe this property to a volatile oil existing 

 in them ; but it is not less certain, that others 

 emit a smell only when they undergo change 

 or decomposition. 



Arsenic and arsenious acid are both quite 

 inodorous. It is only during their oxidation 

 that they emit their characteristic odour of 

 garlic. The oil of the berries of the elder- 

 tree, many kinds of oil of turpentine, and oil 

 of lemons, possess a smell only during their 

 oxidation or decay. The same is the case 

 with many blossoms; and Geiger has shown, 

 that the smell of musk is owing to its gradual 

 putrefaction and decay. 



It is also probable, that the peculiar odor- 

 ous principle of many vegetable substances 

 is newly formed during the fermentation of 

 the saccharine juices of the plants. At all 

 events, it is a fact, that very small quantities 

 of the blossoms of the violet, elder, linden, 

 or cowslip, added to a fermenting liquid, are 

 sufficient to communicate a very strong taste 



and smell, which *he addition of the water 

 distilled from a quantity a hundred times 

 greater would not effect. The various kinds 

 of beer manufactured in Bavaria are dis- 

 tinguished by different flavours, which are 

 given by allowing small quantities of the 

 herbs and blossoms of particular plants to 

 ferment along with the wort. On the Rhine, 

 also, an artificial bouquet is often given to 

 wine for fraudulent purposes, by the addition 

 of several species of the sage and rue to the 

 fermenting liquor; but the fictitious perfume 

 thus obtained differs from the genuine aroma, 

 by its inferior durability, and by being gra- 

 dually dissipated. 



The juice of grapes grown in different 

 climates differs not only in the proportion 

 of free acid which it contains, but also in 

 respect of the quantity of sugar dissolved in 

 it. The quantity of azotised matter in the 

 juice seems to be the same in whatever part 

 the grapes may grow ; at least no difference 

 has been observed in the amount of yeast 

 formed during fermentation in the south of 

 France, and on the Rhine. 



The grapes grown in hot climates, as well 

 as the boiled juice obtained from them, are 

 proportionally rich in sugar. Hence, during 

 the fermentation of the juice, the complete 

 decomposition of its azotised matters, and 

 their separation in the insoluble state, are 

 effected before all the sugar has been con- 

 verted into alcohol and carbonic acid. A 

 certain quantity of the sugar consequently 

 remains mixed with the wine in an unde- 

 composed state, the condition necessary for 

 its further decomposition being absent. 



The azotised matters in the juice of grapes 

 of the temperate zones, on the contrary, are 

 not completely separated in the insoluble 

 state, when the entire transformation of the 

 sugar is effected. The wine of these grapes, 

 therefore, does not contain sugar, but vari- 

 able quantities of undecomposed gluten in 

 solution. 



This gluten gives the wine the property 

 of becoming spontaneously converted into 

 vinegar, when the access of air is not pre- 

 vented. For it absorbs oxygen and becomes 

 insoluble; and its oxidation is communi- 

 cated to the alcohol, which is converted into 

 acetic acid. 



By allowing the wine to remain at rest in 

 casks with a very limited access of air, and 

 at the lowest possible temperature, the oxida- 

 tion of this azotised matter is effected with- 

 out the alcohol undergoing the same change, 

 a higher temperature being necessary to 

 enable alcohol to combine with oxygen. As 

 long as the wine in the stilling-casks de- 

 posits yeast, it can still be caused to ferment 

 by the addition of sugar, but old well-layed 

 wine has lost this property, because the con- 

 dition necessary for fermentation, namely, a 

 substance in the act of decomposition or 

 putrefaction, is no longer present in it. 



In hotels and other places where wine is 

 drawn gradually from a cask, and a propor- 

 tional quantity of air necessarily introduced, 



