CHEMISTEY OF HORTICULTUEE. 
197 
absorbs it, it cannot be long preserved. Professor Brande says that, "Thus obtained, it is 
a colourless gas of a slightly sour odour, considerably heavier than atmospheric air, its 
specific gravity being about as 1*52 to 1-000. Compared with hydrogen, its specific 
gravity is as 23 to 1. The liquefaction of this gas has been effected by a very delicate but 
dangerous operation, which was described in the Phil. Trans, of 1823. 
As carbonic acid gas is formed by the combustion of any substances that contain carbon, 
the oxygen of the air combining with the burning carbonous matter, so it is developed 
during fermentation, putrefaction, and decay, by a slow natural process of combustion ; it 
is not, therefore, at all surprising that, to a certain extent, it should be always present in 
atmospheric air, though its volume must vary in proportion to the quantity extricated 
during the several generating processes. Carbonic acid is evolved from the lungs of every 
breathing animal ; and perhaps, if we attach credit to the facts stated by several physiolo- 
gists, " from the coloured parts of flowers, both by day and night, and from the green parts 
of plants during the night." The odour of flowers is offensive and deleterious to many 
persons, and as the effluvium exhaled is material, although indiscernible, it is more than 
probable that the carbonic acid, said to be evolved, is combined with the vapour of essential 
oil, specific to each flower ; and if so the presence of hydrogen must be included. But 
though any hypothesis, or mere supposition must be received with jealousy, yet it is in 
the power of every reader to establish the fact that carbonic acid is expelled from the lungs ; 
for, if any one will force his breath through a straw or reed into clear water, he will soon 
perceive that the bubbles produce almost immediately a faint haziness, which gradually 
increases till the liquid becomes quite white, from the combination of pure lime in the 
water with the carbonic acid of the breath, the one neutralising the other, and constituting 
chalk, which is carbonate of lime. The subject is of deep interest to the gardener, who 
is often perplexed by the hardness, and ungenial qualities of the water, which he may be 
constrained to employ during the absence of rain, or of soft water streams. It has lately 
come under serious and scientific notice, and will require our close investigation in a 
future article. 
Lime water properly made affords a deUcate test of the presence of carbonic acid in the 
air. Lime of the best quality and when fresh from the kiln, is soluble in water, though 
only to the limited extent of about one ounce in 4| gallons, and therefore a great error is 
committed when persons throw a quantity of lime into a small vessel in order to insure a 
strong solution wherewith to remove the mosses, lichens, &c., which are apt to disfigure the 
stems and branches of currant and other shrubs. Lime water, kept for a time in a close 
stopped bottle, becomes quite clear; when so, if a small quantity be exposed in a saucer in 
the open air, a thin coat or pellicle will speedily form on the surface, and this, if disturbed, 
will fall to the bottom as flakes. These being tested with a few drops of muriatic acid, 
or strong vinegar, will effervesce with a hissing sound, and the extrication of frothy bubbles 
of carbonic acid ; proving that the pure lime, in solution, has attracted and combined with 
the carbonic acid floating in the atmosphere. 
Hot lime simply exposed, under cover of an open shed, will gradually lose its form and 
crumble into a white powder. This first effect demonstrates the presence of water in the 
air, and so far the lime becomes simply air-slacked, retaining for a time its alkaline 
causticity : but by protracted exposure, it combines with the floating carbonic acid, and 
gradually is converted to inert chalk, or true carbonate of lime. 
Watery vapour must, of necessity, be always present, in consequence of the immensity 
of the volume evaporated from the surface of the earth. " When Dr. Halley was at St. 
Helena, he made a variety of experiments on the evaporation from the surface of the sea, 
and found that ten square inches of water evaporated one cubic inch in twenty-four hours ; 
or that a surface of a square mile would evaporate daily 6914 tons. It is calculated that 
the Mediterranean Sea evaporates daily no less than 5280 millions of tons. The total 
average quantity of water evaporated from the whole surface of the earth is calculated by 
Br. Thompson, to amount annually to 94,450 cubic miles." Even in the driest seasons, 
when the ground was parched and appeared void of moisture, when it cracked and formed 
rents and fissures, it was ascertained by experiment, with a glass placed on the ground, that 
i 
