with the loss of their leaves. Hence, in vegetation carried on in smoky atmospheres, the plants are rarely killed altogether, but merely blighted for a season. Accordingly, in spring, vegetation recommences with its accustomed luxuriance, and as in many situations there is, at that season and during the summer, a considerable diminution in the number of coal fires, there will be a proportionate decrease in the production of sulphurous acid gas; and, consequently, less injury will be done to plants during that season. In winter, too, when coal fires mostly abound and gas is most abundantly generated, deciduous plants are protected from its noxious operation by suspension of their vegetating powers; but the leaves of evergreens, which continue to grow through that season, are constantly exposed to its action when present in its greatest intensity. Accordingly, in many of the suburban districts round London, especially in the course of the river where new manufactories are constantly rising up, the atmosphere is so highly charged with noxious matters, that many deciduous plants and almost all evergreens cease to flourish, or exhibit only a sickly vegetation.

"In an interesting biographical sketch of his late lamented friend Dr. Turner, Professor Christison confirms, by subsequent experience, the opinion formerly given respecting the noxious operation of the sulphurous or muriatic acid gas upon plants. He describes their action as so energetic, that, in the course of two days, the whole vegetation of various species of plants may be destroyed by quantities so minute as to be altogether inappreciable by the senses. On two occasions he was able to trace the identical effects of the same kind of works (the black ash manufactory) on the great scale, which his friend and

In one instance the

himself witnessed in their researches. devastation committed was enormous, vegetation being for the most part miserably stunted, or blasted altogether, to a distance of fully a third of a mile from the works, in the prevailing direction of the wind. Against the evils arising from such a vitiated atmosphere, the plan of Mr. Ward provides effectual protection, as the success of his own establishment amply demonstrates.”

The correctness of the above observations of Messrs. Turner and Christison, as to the effects of sulphurous and muriatic acid gases upon plants, cannot for one moment be doubted; and that plants suffer when exposed to a direct current of these gases, before there is time for diffusion through surrounding space, is equally matter of fact; but I contend, that it yet remains to be proved that there exists generally, in the atmosphere of London or other large cities, such a proportion of these noxious gases as sensibly to affect vegetation. We shall find in the windows of shops and small houses, in numerous parts of London, hundreds of geraniums and other plants, growing very well and without any crisping or curling of their leaves, care being taken in these instances to keep the plants perfectly clean, and free from soot; and it is certain, that although my cases can and do exclude the fuliginous portion of the atmosphere, and certainly protect the plants from the effects of any direct current of hurtful airs, they cannot exclude that portion which becomes mixed with the atmosphere. This subject is one of the highest importance, and intimately connected with the well-being of everything that hath life, whether vegetable or animal. It cannot, however, be understood, without reference to, and a full explanation of, the law which regulates the dif

C

fusion of gases, a law constantly in action under all circumstances, and without the beneficent operation of which, vegetable as well as animal life would suffer greatly in large towns, and a cellar in St. Giles's quickly become a Grotto del Cane.

"If we take two vessels, and fill one with carbonic acid gas, and the other with hydrogen, (their weights respectively being as 22 to 1), and then place the light gas perpendicularly over the other, effecting a communication between the vessels by means of a tube not larger in diameter than a human hair, the two gases will immediately begin to mix, and after a short interval will be found equally distributed between both vessels. If the upper vessel be filled with oxygen, nitrogen, or any other gas, the same phenomena will ensue; the gases will be found, after a short time, to be in a state of mixture, and at last there will be equal portions of each in both vessels. The permeability of animal membranes by gases has been fully proved by the researches of Drs. Faust and Mitchell. It fully appears from their experiments that animal membranes, both in the living and dead subject, both in and out of the body, are freely penetrated by gaseous matter; that the phenomena of endosmose and exosmose, observed in liquids by Dutrochet, are likewise exhibited by gases. If a glass full of carbonic acid be closed by an animal membrane, or sheet of caoutchouc, and be then exposed to the atmosphere, a portion of air will pass into the glass and some of the confined gas escape from it; and if the experiment be reversed, by confining air in the glass, which is then placed in an atmosphere of carbonic acid, the latter passes in and the former out of the glass. Similar phenomena ensue with other gases; so that when any

two gases are separated by a membrane, both of them pass through the partition. But though all gases pass through membranous septa, they differ remarkably in the relative rapidity of transmission. Thus, while a volume of carbonic acid requires 5 minutes to pass through a membrane, the same volume of oxygen requires 113, and a much greater time is required for nitrogen. Hence, when a bladder full of air is surrounded by carbonic acid, the latter enters faster than the former escapes, and the bladder bursts; but on reversing the conditions of the experiment the bladder becomes flaccid, because the carbonic acid within passes out more rapidly than the exterior air enters. The transmission of gases in some of these experiments takes place in opposition to a pressure equal to several atmospheres."*

In order to ascertain what the degree of circulation through the substance of the membrane in some closed jars might have been, Dr. Daubeny+ removed from one of the jars the plants and vegetable mould it contained, and then substituted for them about an equal amount of dry sand. He next passed through the vessel a current of oxygen, until the volume of air within contained no less than 77 per cent. of that gas. The air was examined again, at 4 p.m., after an interval of three hours from the period of the first experiment, when it was found to have lost 4 per cent. of oxygen. At 8 the next morning it was found to contain only 63 per cent. of oxygen, having diminished, in 16 hours, 10 per cent. After having been exposed all day to air and light, and examined at 8 p.m.,

*Turner's Elements.

+ Dr. Daubeny's Report to the Meeting of the British Association, at Liverpool.

the oxygen was found to amount to only 45 per cent., having diminished, in 12 hours, 18 per cent. During the next night it had diminished, in 12 hours, only 6 per cent., the amount of oxygen the next morning being 38 per cent. During the next day it had lost 7 per cent., containing at 8 p.m. 31 per cent. The next night the diminution was only 2 per cent., and on the succeeding day 3 per cent. The following night the diminution was 1 per cent., the amount of oxygen being 24 per cent. only. During the day a further diminution of 3 per cent. took place, the air enclosed within the jar being found to contain exactly the quantity of oxygen present in atmospheric air. The following is a tabular view of the results.

June 23. 1 p.m.. amount of oxygen, 177

4 p.m.
8 a.m.

... excess, 56

73

52

............

Thus five days were required to enable the whole excess of oxygen to pass through the substance of the membrane, the diameter of which was three inches, whilst the capacity of the vessel, when the sand had been introduced, was nearly one gallon, so that about three quarts of oxygen and one of nitrogen may be calculated as having been present in the jar at the commencement of the experiment, of which about 4 pints were discharged through the membrane in the course of the five days during which the observations were continued.