1818.] and of Sugar in the Potatoe. 337 



years a favourite pursuit of the chemists, different vegetable 

 principles have been discovered in different parts of plants ; but 

 hitherto it has not been demonstrated in what state potash occurs 

 in vegetables ; and though it has been found in the ashes of 

 plants, its presence had not been ascertained either in their 

 expressed juices or their decoctions. A set of experiments made 

 upon a certain genus of plants with a view of studying its 

 medical virtues has led to the discovery of potash in these 

 different liquids, and has suggested an easy mode of determin- 

 ing the acid with which the potash was united. 



All the juices (the word juice denotes the liquid expressed 

 from a plant, but not from its fruit), or the decoctions of the 

 different parts of a vegetable, are more or less acid, reddening 

 paper stained blue with litmus, &c. It was necessary to rind a 

 substance which should not only combine with the disengaged 

 acid, but which should have a greater affinity for that acid than 

 potash itself has. The acids which occur most commonly in 

 plants being the carbonic, the tartaric, and the oxalic, it was 

 necessary besides that this substance should form an insoluble 

 salt with each of these acids. Pure magnesia answers this pur- 

 pose completely. If then we agitate in the cold, or boil together 

 a vegetable juice or decoction, and a quantity of pure magnesia, 

 we obtain, after the separation of the deposite, an alkaline 

 liquid, which possesses all the characters of a solution of 

 carbonate of potash. By examining the magnesia in the requisite 

 manner, we can easily determine the acid with which it has 

 combined. (The tartrates and oxalates of magnesia are insoluble, 

 when there is no excess of acid — an excess which it is of 

 importance to avoid.) 



The insolubility of pure magnesia and of a part of the salts 

 which it forms when united with acids, renders the process very 

 accurate, and of very easy execution. 



If the salts contained in the vegetables be sulphates or nitrates, 

 they not only do not redden vegetable blues (because there is no 

 excess of acid present) ; but magnesia is not capable of decom- 

 posing them. This is the case with borage, &c. 



This discovery, besides its importance in the analysis of plants, 

 facilitates the means of judging of the quantity of potash con- 

 tained in vegetable juices. Hereafter, incineration will not be 

 necessary in order to obtain that potash. The choice which a 

 philosophical society in Holland made of this problem as a prize 

 question (in 1817), proves the interest with which it was viewed. 



Second Discovery. — In consequence of a careful analysis, 

 Vauquelin drew as a conclusion, that potatoes are composed of 

 starch, of parenchyma, of a peculiar animal matter, and of 

 certain salts. These different substances did not explain the 

 cause of the spirituous fermentation which they undergo, if they 

 are exposed, sufficiently diluted with water, and mixed with a 

 little barley meal to the requisite temperature. Hence it has 



Vol. XII. N° V. Y 



