SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



181 



stance is neither acid nor resinous; but is 

 either saccharine or mucilaginous, or 

 analagous to woody fibre or starch, when- 

 ever the oxygen or hydrogen* in it are in 

 the same proportions as in water." 



New experiments upon other vegeta- 

 ble substances, besides those examined 

 by MM. Gay Lussac and Thenard, are 

 required before these interesting conclu- 

 sions can be fully admitted. Their re- 

 searches establish, however, the close 

 analogy between several vegetable com- 

 pounds differing in their sensible quali- 

 ties, and combined with those of other 

 chemists, offer simple explanations of 

 several processes in nature and art, by 

 which different vegetable substances are 

 converted into each other, or changed 

 into new compounds. 



Gum and sugar afford nearly the same 

 elements by analysis: and starch differs 

 from them only in containing a little more 

 carbon. The peculiar properties of gum 

 and sugar must depend chiefly upon the 

 different arrangement, or degree of con- 

 densation of their elements; and it would 

 be natural to conceive from the composi- 

 tion of these bodies, as well as that of 

 starch, that all three would be easily con- 

 vertible one into the other; which is ac- 

 tually the case. 



At the time of the ripening of corn, 

 the saccharine matter in the grain, and 

 that carried from the sap vessels into the 

 grain, becomes coagulated, and forms 

 starch. And in the process of malting, 

 the converse change occurs. The starch 

 of grain is converted into sugar. As 

 there is a little absorption of oxygen, and 

 a formation of carbonic acid in this case, 

 it is probable that the starch loses a little 

 carbon, which combines with the oxygen 

 to form carbonic acid; and probably the 

 oxygen tends to acidify the gluten of the 

 grain, and thus breaks down the texture 

 of the starch, gives a new arrangement 

 to its elements, and renders it soluble in 

 water. 



Mr. Cruikshank, by exposing syrup to 

 a substance named phosphuret of lime, 

 which has a great tendency to decompose 

 water, converted a part of the sugar into 

 a matter analogous to mucilage. And 

 M. Kirchhoff, recently, has converted 

 starch into sugar by a very simple pro- 



cess, that of boiling in very diluted sul- 

 phuric acid. 



The proportions are 100 parts of starch, 

 400 parts of water, and 1 part of sulphu- 

 ric acid by weight. This mixture is to 

 be kept boiling for 40 hours; the loss of 

 water by evaporation being supplied by 

 new quantities. The acid is to be neu- 

 tralized by lime; and the sugar crystal- 

 lized by cooling. This experiment has 

 been tried with success by many persons. 

 Dr. Tuthill, from a pound and a half of 

 potato starch, procured a pound and a 

 quarter of crystalline brown sugar; 

 which he conceives possessed properties 

 intermediate between cane sugar, and 

 grape sugar. 



It is probable that the conversion of 

 starch into sugar is effected merely by 

 the attraction of the acid for the elements 

 of sugar; for various experiments have 

 been made, which prove that the acid is 

 not decomposed, and that no elastic mat- 

 ter is set free; probably the color of the 

 sugar is owing to the disengagement, or 

 new combination of a little carbon, the 

 slight excess of which, as has been just 

 stated, constitutes the only difference 

 perceptible by analysis between sugar 

 and starch. 



M. Bouillon la Grange, by slightly 

 roasting starch, has rendered it soluble in 

 cold water; and the solution evaporated 

 afforded a substance, having the charaters 

 of mucilage. 



Gluten and albumen differ from the 

 other vegetable products, principally by 

 containing azote. When gluten is kept 

 Ions: in water it undergoes fermentation; 

 ammonia (which contains its azote) is 

 given off with acetic acid: and a fatty 

 matter, and a substance analogous to 

 woody fibre remain. 



Extract, tannin, and gallic acid, when 

 their solutions are long exposed to air, 

 deposite a matter similar to woody fibre; 

 and the solid substances are rendered 

 analogous to woody fibre by slight roast- 

 ing; and in these cases it is probable that 

 part of their oxygen and hydrogen is 

 separated as water. 



AH the other vegetable principles dif- 

 fer from the vegetable acids, in contain- 

 ing more hydrogen and carbon, or less 

 oxygen; many of them therefore are 



