Physical research has proved to us that all bodies have peculiar powers, by which they condense with varying degrees of force gases and vapours upon their surfaces; every body in nature may, indeed, be regarded as forming its own peculiar atmosphere. To this power, in all probability, does catalysis belong. Different views have, however, prevailed on this subject, and Dr. Lyon Playfair[207] argues that the catalytic force is merely a modified form of chemical affinity, exerted under peculiar conditions. Whatever may be the power producing chemical change, it acts in conformity with some fixed laws, and in all its transmutations, an obedience to a most harmonious system is apparent. It is curious to observe the remarkable character of many of these natural transmutations of matter, but we must content ourselves with a few examples only. For instance:-- Sugar, oxalic acid, and citric acid are very unlike each other, yet they are composed of the same elements; the first is used as a general condiment, the second is a destructive poison, and the third a grateful and healthful acid: sugar is readily converted into oxalic acid, and in the process of ripening fruits nature herself converts citric acid into sugar. Again, starch, sugar, and gum would scarcely be regarded as alike, yet their only difference is in the mode in which carbon, hydrogen, and oxygen combine. They are composed of the same principles, in the following proportions:-- Carbon. Hydrogen. Oxygen. Starch 12 10 10 Sugar 12 11 11 Gum 12 11 11