Carbonic acid consists of carbon and oxygen only, combined together in the proportion of 28 of the former to 72 of the latter, or 100 lbs. of carbonic acid contain 28 lbs. of carbon and 72 lbs. of oxygen. 2. HUMIC AND ULMIC ACIDS.—The soil always contains a portion of vegetable matter (called _humus_ by some writers), and such matter is always added to it when it is manured from the farm-yard or the compost heap. During the decay of this vegetable matter, carbonic acid, as above stated, is given off in large quantity, but other substances are also formed at the same time. Among these are the two to which the names of humic and ulmic acids are respectively given. They both contain much carbon, are both capable of entering the roots of plants, and both, no doubt, in favourable circumstances, help to feed the plant. If the common soda of the shops be dissolved in water, and a portion of a rich vegetable soil, or a bit of peat, be put into this solution, and the whole boiled, a brown liquid is obtained. If to this brown liquid, spirit of salt (muriatic acid) be added till it is sour to the taste, a brown flocky powder falls to the bottom. This brown substance is _humic_ acid. But if in this process we use spirit of hartshorn (liquid ammonia), instead of the soda, _ulmic_ acid is obtained. These acids exist along with other substances in the rich brown liquor of the farm-yard, which is so often allowed to run to waste; they are also produced in greater or less quantity during the decay of the manure after it is mixed with the soil, and no doubt yield to the plant a portion of that supply of food which it must necessarily receive from the soil. SECTION III.—OF WATER, AMMONIA, AND NITRIC ACID. 1. WATER.—If hydrogen be prepared in a bottle, in the way already described, and a gas-burner be fixed into its mouth, the hydrogen may be lighted, and will burn as it escapes into the air. Held over this flame a cold tumbler will become covered with dew, or with little drops of water. This water is _produced_ during the burning of the hydrogen; and as it takes place in pure oxygen gas as well as in the open air, this water must contain the hydrogen and oxygen which disappear, or _must consist of hydrogen and oxygen only_. This is a very interesting fact; and were it not that chemists are now familiar with many such, it could not fail to appear truly wonderful that the two gases, oxygen and hydrogen, by their union, should form so very different a substance as water is from either. It consists of 1 of hydrogen to 8 of oxygen, or every 9 lbs. of water contain 8 lbs. of oxygen and 1 lb. of hydrogen.