196 DECOMPOSITION BY CONTACT. 



of chemical affinity, which he has distinguished as the Cata- 

 lytic force, and the effect of its action as Catalysis (from Kara 

 downwards, and Xvw, I unloosen). A body in which this power 

 resides, resolves others into new compounds,- merely by con- 

 tact with them, or by an action of presence, as.it has been 

 termed, without gaining or losing anything itself. Thus an 

 acid converts a solution of starch (at a certain temperature), 

 first into gum, and then into sugar of grapes, although no 

 combination takes place between the elements of the acid, and 

 those of the starch, the acid being found free and undiminished 

 in quantity, after effecting the change. The same mutations 

 are produced in a more remarkable manner by the presence 

 of a minute quantity of a vegetable principle diastase^ allied 

 in its general properties to gluten, which appears in the ger- 

 mination of barley and other seeds, and converts their starch 

 into sugar and gum, which being soluble, form the sap that 

 rises into the germ, and nourishes the plant. This example 

 of the action of a catalytic power in an organic secretion is 

 probably not the only one in the animal and vegetable king- 

 doms, for it is not unlikely that it is by the action of such a 

 force that very different substances are obtained from the 

 same crude material by different organs. In animals this 

 crude material, which is the blood, flows in the uninterrupted 

 vessels, and gives rise to all the different secretions ; such as 

 milk, bile, urine, &c. without the presence of any foreign 

 body which could form new combinations. A beautiful in- 

 stance of an action of catalysis has been traced by Liebig and 

 Wohler in the chemical changes which the bitter almond 

 exhibits. The application of heat and water to the almond, 

 by giving solubility to its emulsin or albuminous principle, 

 enables it to act upon an associated principle, amygdalin, of 

 a neutral character, which then furnishes bodies so unlike 

 itself as the volatile oil of almonds, and the hydrocyanic, oxalic 

 and formic acids. The action of yeast in fermentation is a 

 more familiar illustration of a similar power. The presence 

 of that substance, although insoluble, is sufficient to cause the 

 resolution of sugar into carbonic acid gas and alcohol, a de- 

 composition which can be effected by no other known means. 

 Changes of this kind, although 'most frequent in organic com- 

 pounds, are not confined to them. The peroxide of hydrogen, 

 discovered by Thenard, is a body of which the elements are 



