If the whole be now introduced into a large basin of water, the tumbler being held in the left hand, the cup and saucer may be removed from beneath. The saucer may then be inverted and introduced with its under side into the mouth of the tumbler, which may thus be lifted out of the water and restored to its upright position, the saucer serving the purpose of a cover. By carefully removing this cover with the one hand, a lighted taper may be introduced by the other. It will then be seen that the taper is extinguished by this air, and that no other effect follows. Or if a living animal be introduced into it, breathing will instantly cease, and it will drop without signs of life. This gas possesses no other remarkable property. It is a very little lighter than common air, and is known to exist in large quantity in the atmosphere only. Of the air we breathe it forms nearly four-fifths of the entire bulk. These three gases are incapable of being distinguished from common air, or from each other, by the ordinary senses; but by the aid of the taper they are readily recognised. Hydrogen extinguishes the taper, but itself takes fire; nitrogen simply extinguishes it; while in oxygen the taper burns with extraordinary brilliancy and rapidity. Of this one solid substance, carbon, and these three gases, hydrogen, oxygen, and nitrogen, all the organic part of vegetable and animal substances is made up. Into these substances, however, they enter in very different proportions. Nearly one-half the weight of all vegetable productions which are gathered as food for man or beast—in their dry state—consists of carbon; the oxygen amounts to rather more than one-third, the hydrogen to little more than five per cent., while the nitrogen rarely exceeds two and a half or three per cent. of their weight. This will appear from the following table, which exhibits the actual constitution by analysis of some varieties of the more common crops when perfectly dry. Carbon. Hydrogen. Oxygen. Nitrogen. Ash. Hay, 458 50 387 15 90 Potatoes, 441 58 439 12 50 Wheat Straw, 485 52 389½ 3½ 70 Oats, 507 64 367 22 40 These numbers represent the weights of each element in pounds, contained in 1000 lbs. of the dry hay, potatoes, &c.; but in drying by a gentle heat, 1000 lbs. of hay from the stack, lost 158 lbs. of water, of potatoes wiped dry externally 722 lbs.,[1] wheat straw 260 lbs., and oats 151 lbs. [1] Both potatoes and turnips contain about four-fifths of their weight of water, or five tons of either of these roots contain nearly four tons of water. SECTION III.—OF THE MEANING OF CHEMICAL COMBINATION.