84

CHEMICAL CHANGES.

are tlie vegetable alkalies, as they are called; these arecompounds of oxygen, hydrogen, carbon, and nitrogen,which possess the power of combining with acids andforming neutral salts ; they are of course destroyed byheat, and whilst burning give off the usual results ofthe combustion of organic matter—water, carbonic acid,and ammonia.

252. It is certainly a very surprising fact, that somany different substances should be formed by thecombination of the same elements in different propor-tions. Nothing can well be more dissimilar than oiland sugar, flax and starch ; yet it is easily proved thatthey all consist of the same elements.

25S. The knowledge of this might naturally lead oneto suppose that, if the whole difference between suchsubstances consists in the relative proportions of carbon,oxygen, and hydrogen which they contain, it might bepossible, by some chemical operation, to take away asmall portion of carbon or hydrogen, and thus, byaltering the relative proportions of the elements, toconvert starch into gum, or flax into starch. Now thiscan really be done; and strange as it may appear, it isnevertheless true, that, by very simple means, it is easyto change gum, starch, and lignin, &c. into each other.

254. Sugar, spirit of wine, and vinegar, consist ofthe same elements: they contain oxygen, carbon, andhydrogen; but the elements are united in different pro-portions ; the properties of these three substances are asopposite as they well can be, and yet the whole differ-