52.

secured in the masonry of the pillars." (Figs. 7 and 8.)

The platform was made in a different method from those of our Calcutta bridges, as will be understood by the following explanation:

"From the short links set between the centré plates of the shackles (of the main chains), are suspended alternately from ties, 74 vertical round rods, 1 inch in diameter, connected to a short link (fig. 6,) by a 1-inch round bolt passing through it, and the socket at the upper end of the bar; at their lower ends the rods have eyes, through which doubled loops of iron pass (3 and 4) for sustaining the flat bars or girders, set on their edges, and proceeding from one end to the other on both sides of the bridge.

"The flat bars, 4 inches broad by inch thick, and in lengths of 15 feet, are joined at their ends by nicely turned bolts, passing through bored holes 2 inches in diameter; they are adjusted in their height by double wedges resting on holders that connect the sides of the loops together. The girders are also adjustable in their lengths; the bars that enter the masonry have their ends made broader than the rest of the bars, in which are long openings 2 inches broad to receive wedges (figs. 10 and 11).

"Eight timbers in an upright position are set in the masonry of the pillars, having upright grooves or spaces cut through them, and faced with thick plates of iron; through two of these beams each end bar passes, and may be wedged on either side of the timber towards the land, as occasion may require: thus is the whole length of girder drawn more or less to either end of the bridge, and also rendered exceedingly tight and steady. The grooves in the timbers towards the river, being about 4 inches longer than the breadth of the bars, permit them to adapt themselves to their proper directions when drawn lengthwise, by the wedges acting against the landward beams; by this means the bars have sufficient play to adapt themselves to the motion of the platform, and all jirks at the pillar obviated.

"Thirty-seven double-joists, 12 feet long, are (having their ends notched below for the purpose) laid on the girders; their centres,

6 double main chains, joists, and bolts 74 vertical rods with joints, bolts, &c.

5 feet apart, correspond exactly with the vertical rods that pass through them: the joists are composed each of two cheeks, a foot in depth and 3 inches thick, separated at intervals by four blocks of wood of the same height and thickness, all firmly put together with bolts, screws, and nuts; two cleats are nailed to each end of the joist on their under sides, whose ends fit flat against the girder and keep all steady.

"Planks, 16 feet in length, running longitudinally, each plank stretching over three paces, and regularly disposed as to their joints, are spiked down on the joists: in a direction across these, and upon them other planks are spiked down, their lengths being the same as the breadth of the platform. The planks are all embedded in a composition of resin boiled in linseed oil, which in laying on is mixed with ashes. The lower planks are three and the upper ones 3 inches thick; they are only six inches broad to prevent warping, and have two strong squareheaded spikes passing through them near their edges at every crossing of the uppers over the lower planks: their joints are: clinched below the platform, to accomplish which 16,370 spikes weighing a ton and a half were used; thus the platform has been rendered extremely strong and firm.

"The better to secure the sides of the platform and ends of the timbers from the weather, a cornice or moulding of wood is nailed along the outside.

"The hand-rail is trussed, and consists of iron pillars or stanchions; diagonal braces of iron, and a stout wooden rail running from i end to end of the platform: the whole put together with screws and nuts, and adjusting screws for setting up or tightening the dia gonal braces whenever required. (Fig. 10)

"The rise in the platform is as before stated, 9 inches, but the curve of the handrail is only 3 inches; to effect which, the stanchions which support the rail are of varying lengths, the rail being 4 feet 6 inches above the platform at its connection with the masonry, but only 4 feet in the centre of the bridge.'

The following are the weights of the chains, rods, and materials of the plate form:

Flat bars and bolts

ELECTRO-VEGETATION.

Letter 1V.

Sir,-We propose next to consider how far the operations of light on vegetation are referable to electrical agency. Light extracts moisture in great abundance, and oxygen in degrees proportioned to the quantity of carbonic acid present in the atmosphere from the under surfaces of the leaves when their vegetation is vigorous. It has been supposed that the moisture exhaled from plants bears a near analogy to the perspiration of animals, and might in like manner be promoted by exposure to heat in the absence of light. It appears, however, to be a process altogether dependent on the action of light; for if two plants be placed under the sun's rays, the one open to their action, the other with an opaque covering, which, while it excludes their light, admits their heat in an equal or greater degree, a glass vessel being placed over both for the purpose of receiving moisture, the first will be quickly suffused with pure water standing in drops, while the latter will remain perfectly dry. This exhalation, therefore, is not a result of any

internal process passing in plants, by which heat is generated within them and thrown off in the form of vapour, but is the pure action of the light combining with the aqueous particles of the sap, and transforming them into gas. It proceeds from the under surface of the leaf, which in general is more porous and less resinous and polished than the upper surface: a fact which may be observed by inverting two glass vessels, the one on the back, and the other on the face of a leaf; the first, if exposed to the sun, will be quickly suffused with moisture, while the other will receive no moisture whatever. This extraction of one of the elements of the sap by the pure agency of

light abstracted from heat, bears a strong analogy to that of electricity; and indicates that this is the principle by which the decomposition is effected; which though partial, as not being sufficiently. intense to resolve the water into its elements, is yet the result of such a degree of electric influence as is requisite for the occasion, rendering the sap more dense for its descending course, and removing many of those aqueous particles which by conducing to its fluidity and levity, greatly aided in its ascending direction.

Oxygen gas is now well known to be extracted from the leaves of plants by the agency of light. Of this fact I have satisfied myself, by observing that the production and increase of gas in glass vessels inverted over fresh leaves merged in water, depended wholly on their exposure to the light. In my experiments the increase was slow, in consequence of my not being aware at the time of the necessity of the presence of carbonic acid in the water to the production of the oxygen. I, however, found that the gas thus procured was pure oxygen, which, though slightly diminished during the night season, upon the whole furnished me with a sufficient supply for a satisfactory experiment, proving it to be oxygen in a state of purity. My indefatigable friend, Mr. Weekes, has arrived at the same conclusion, by experiments made by means of his newly-invented pneumatic apparatus, described in your pages, on plants while remaining in air as their natural element. But I do not know whether he was aware, or is prepared to admit, the necessary presence of carbonic acid in the atmosphere, in order to the production of oxygen, which, however, appears in several very decisive experiments of Sir H. Davy, related in his Elements of Chemistry; and is confirmed by Woodhouse, who "drew from his ex

periments the conclusion that the change produced by the vital action of the vegetable, excited by solar light, is the decomposition of carbonic acid, and consequent evolution of oxygen." Heat, in the absence of light, is known to be wholly inefficient in the production of this process. We have, then, in this case, an evidence of several chemical effects to which light, as distinguished from heat, is essential;-the decomposition of carbonic acid, attended probably with a deposition of the carbon in the leaf, or its union with some other substance contained in it, (and the extraction of its oxygen, which probably, by entering into combination with the particles of light, escapes in the form of gas. Is it not reasonable to conclude, that the agent in the produc tion of these chemical changes is identified with the electric fluid; and that thus the solar beams, instead of accumulating in the atmosphere in the form of electricity, and generating storms or hurricanes, are converted into the means of maintaining the vital principle in animals? By this means one portion of that fluid, which is so copiously dispensed from the sun in the summer season, may be conceived as being both neutralized, or prevented from proving the occasion of much disorder and injury in the system of nature,-and converted into the great essential of animal vitality; while a much larger portion is neutralized by a similar combination with the particles of water, and by forming clouds and rain, both mitigates the heat, and again imparts nurture to vegetation.

These rays, if suffered to float in the atmosphere without entering into these combinations, would either derange the harmony of nature by electrical concussions far exceeding any which fall under our experience, or by rarifying and raising the temperature of the atmosphere in the like unexampled and intolerable degrees, render it in these respects, as well as by the probable absence of oxygen, unfit for the purposes of animal existence.

The following experiment will serve to show the necessary presence of carbonic acid in an atmosphere of water to the production of oxygen gas. Some water, out of which the air had been boiled exhibited no gas from a sprig of vineleaves, exposed to the sun's rays; but on being impregnated with carbonic acid, and placed in the same circumstances,

large and copious globules appeared on the back of the leaves; and on burning a taper in the gas collected from them, the light was extremely brilliant. This experiment has been repeated in the presence of several persons who were fully satisfied with the result. By many trials I also find that in proportion as the oxygen gas is produced, rhe carbonic acid disappears from the water, and indeed that it is withdrawn from it by the action of the leaves in the absence of light, and during the night season, though no oxygen is produced but on exposure to the solar rays. Having placed a fresh leaf in some pump water which I had found to yield globules to fresh leaves freely when exposed to light, and excluded the light from them for about an hour, I was surprised to observe that on removing the covering, and suffering the leaf and water which had been in darkness to remain a considerable time under the beams of the sun, no globules whatever appeared; and from subsequent observations on sprigs of leaves in jars of the same water, I discovered that the carbonic acid was as completely removed by the action of the leaves, though remaining in total darkness, as if it had been expelled by boiling; no globules appearing in the jars of water which had been confined with sprigs of vine-leaves for about ten hours, provided those leaves were removed and fresh leaves were introduced: whereas when the same leaves remained in it, which, on being wetted by lime water, remained transparent, they, after having been exposed a few hours to the light, began to yield globules; showing that having im bibed the carbonic acid while in darkness, they now yielded its oxygen from the pores under the action of light.

That solar light extracts oxygen from "the leaves of plants, their green shoots or branches, or even the entire vegetable under water," was the observation of Ingenhouz; and he found also that "its production was considerably dependant on the nature of the water under which the vegetable was immersed. It has been since shown by the experiments of Sennibier, Woodhouse, and Saussure, that it is much connected with the presence of carbonic acid; so that in water entirely free from this, the evolution of oxygen gas is very inconsiderable, while in water impregnated with it it is abundant. It may be inferred, therefore, that it is

principally from the decomposition of the carbonic acid by the pores of the plant, aided by the agency of the light, that the oxygen is evolved."* Here are strong confirmations of the principal facts which I have adduced, excepting that these distinguished philosophers appear to have been of opinion that a small portion of the oxygen might proceed from the plant itself in a more direct manner; and that they make no mention of the action of the leaves, by imbibing the carbonic acid in the absence of light. As this particular, being new and wholly unanticipated by me, strongly attracted my attention, I repeated my observations with care, and can entertain no doubt whatever of the fact. I cannot help regarding it as a very satisfactory evidence of the useful operation of plants during the night season, if we admit that they exercise the same function of attaching carbonic acid to their substance under their ordinary circumstances surrounded by air, which they manifest when immersed in water. That they do exercise this function in air, appears from the experiments of Davy and Woodhouse, above alluded to; and as it is probable that a function which appears to be natural to them is exerted with the greatest perfection in their proper element, it seems reasonable to conclude that they are very operative in removing from the atmosphere the deleterious gas which is exhaled from the lungs of animals, by night as well as by day, although they are indebted to the assistance of solar light in evolving the oxygen from the carbon, which now enters permanently into their substance.

Mr. Weekes informs me, as one of the results of his experiments with his new pneumatic apparatus, that healthy plants do yield carbonic acid occasionally, but not in sufficient quantities to affect the conclusion that oxygen gas is their general aerial produce. It has been the opinion of some physiologists that they absorb oxygen, and give out carbonic acid during night. I admit that the oxygen which has been given out during the day sustains some diminution in the night, when the experiment is made over water with the leaves imerged in it; but this is probably to be ascribed to the absorption of the water. It evidently requires a due regard to proportions in introducing

* See Murray's Chemistry, vol. ii. p. 288, &c.

carbonic acid to the action of plants. From one of my experiments, it appeared that two vine-sprigs, each with several leaves in a jar of water from which the air had been withdrawn by the previous action of leaves in it, became beautifully illumined with globules of oxygen, by introducing about one-fourth of its bulk of carbonic acid into the water. Hence it is probable that a very small proportion of this gas diffused, through the air is best suited to the absorbent property of the plants; and perhaps that minute proportion in which it is ordinarily found in the atmosphere, is that which is best suited for their action in that element. Priestley found that when mixed with it in the proportions of from one-half to one-eighth, the plant confined in this mixed atmosphere was almost instantly destroyed; and I have found the leaves apparently injured by too large proportions of the gas in water, under which cireumstances it is highly probable that they occasionally give out a portion of it unaltered, especially when kept beyond their natural or ordinary time in darkness. The results, however, which have fallen under my observation are such as I have above stated; the leaves appearing to attract the carbonic acid from the surrounding medium at all hours, in darkness as well as in the light, and yielding its oxygen under the influence of the solar

rays.

One inference follows from the fact, that the production of oxygen gas from the leaves of plants depends on the presence of carbonic acid in the surrounding medium, which I would hope may prove of some practical utility. Carbonic acid can be procured with considerably more facility than oxygen gas in the ordinary methods; but by impregnating water, containing branches of leaves exposed to the solar rays, with the former of these gases, the latter might be obtained in quantities sufficient for several purposes; as for the recovery of suspended animation in the cases of still-born children, of drowning or other medicinal uses, by being kept in glass bottles well sealed. At any rate, an attention to the requisite quantity of the aerial acid in the water in which vegetables are immersed in order to obtain oxygen gas from them will greatly facilitate success, without which the mere addition of fresh leaves to the water would prove almost wholly un

56

availing; whereas their under surfaces will speedily become finely bedecked with large globules of oxygen, when the water has been impregnated with about one fourth of its bulk of carbonic acid. It will probably occur to the recollection of some of your readers, that "the lady of a physician having a child still-born, all the common means were tried without effect. Recollecting he had a bladder of vital air, with which he was about to make an experiment, the doctor forced this air into the lungs of the infant, when the eddies of its little heart began to play, and the child was restored." Mr. Weekes informed me, that "he had several times actually used oxygen in cases of suspended animation, and with success; and that in the last instance, he had recovered a boy, who was then living four years after, when he had been twenty minutes under water. I find," he adds, "the best method, because the simplest, is to pass an elastic tube into the windpipe, and then inflate the lungs by having the gas diluted with one-third atmospheric air, in bladders ready to attach to the pipe." I have ventured to trespass from the proper object of this paper with these remarks on the means of obtaining oxygen, and this important practical application of the gas, particularly by my friend, in the hope of their conducing to some future usefulness-wishing to obtain all the light and information in my power upon the subject of the electric relations of the soil to vegetation, and the influences of the solar fluid in its connexion with it. I must here close this letter.

KIRKBY SLATE QUARRIES.

Sir, In a former paper on this subject, amongst other circumstances I endeavoured to describe the distinctions between what are called back and foot quarries; and at that time supposing the plane ofcleavage to have some determinate angle with respect to the plane of the stripes, I described the peculiar motion that must have taken place to change the one into the position of the other; I had observed for example, that fig. 1 represents the section of a foot on a vertical north and south plane, and ab the direc tion of the plane of cleavage in that foot

dipping to the south; and that fig. 2 represents the section of a back, also on a vertical north and south plane, ab, being here also the direction of the cleavage dipping to the south. 2 bus dion

Now it will be manifest that if fig. 2 be put into the position of fig. 1 the lines indicating the cleavage in each, will dip in contrary directions:-and supposing a change had taken place in the position of the rock, I could then only account for the dip of the cleavage in both back and feet being to the south, by supposing that one had actually turned round in the manner I have described, and which I may further illustrate by stating that if fig. 2 be drawn on the other side of the same paper on which fig. I is placed, and that both figs. 1 and 2 be then examined by looking through the paper, the dip of the clevage, (the plane of stripes being in the same direction) will then be in the same direction in both.

These appearances had been observed in so many extensive quarries, and both near and at such distances from each other as to have fixed the terms backs and feet, as the means by which those engaged in or about the quarries distinguished the position of the rockin quarries which differed from each other. I had notwithstanding noticed that the angle which the cleavage makes with the stripes was more oblique in the backs than in the feet, but this was not sufficient to lead me to expect so extraordinary a fact as the following.