(67) On some Properties peculiar to Caoutchouc, and their Caoutchouc is a vegetable constituent, the produce of several trees; the most prolific in this substance are, Siphonia Caoutchouc, Urceola elastica, Ficus elastica, &c.; of these the Siphonia Caoutchouc extends over a vast district in Central America, and the caoutchouc obtained from this tree is best adapted for its manufactures. Over more than 10,000 square miles in Assam the Ficus elastica is abundant. The Urceola elastica (which produces the Gintawan of the Malays,) abounds in the islands of the Indian Archipelago. It is described as a creeper of growth so rapid, that in five years it extends 200 feet, and is from 20 to 30 inches in girth. This tree can, without being injured, yield by tapping, from 50 to 60 lb. of caoutchouc in one season. A curious contrast is exhibited in the tardy growth of the tree from which the Gutta Percha is obtained. This tree does not come to its prime in less than from 80 to 120 years. The produce cannot be obtained but by the sacrifice of the tree. It is found in a concrete state between the bark and the wood after the tree has been cut down, and it is in this condition that, having been scraped out, it is sent to our market. When coagulated by evaporation or agitation, caoutchouc separates from the aqueous portion of the sap of the trees which yield it. This solid and fluid cannot afterwards be reunited, any more than butter is capable of mixing with the milk from which it is separated. Caoutchouc is a hydrocarbon. This chemical character belongs to all varieties of the substance, and many other vegetable constituents, though they differ materially in physical qualities. Some specimens are harder than Gutta Percha itself, while others never solidify but remain in the condition of bird-lime or treacle. The process termed the vulcanizing of caoutchouc was discovered by Mr Thomas Hancock in 1843.—A sheet of caoutchouc immersed in melted sulphur absorbs a portion of it, and at the same time it undergoes some important changes * Abstract of a Communication read to the Royal Institution, March 21, 1851. in many of its characteristic properties. It is no longer affected by climatic temperature; it is neither hardened by cold nor softened by any heat which would not destroy it. It ceases to be soluble in the solvents of common caoutchouc, while its elasticity becomes greatly augmented and permanent. The same effect may be produced by kneading sulphur into caoutchouc by means of powerful rollers; or the common solvents, naphtha and spirit of turpentine, may be charged with a sufficient amount of sulphur in solution to become a compound solvent of rubber. In these cases articles may be made in any required forms before heating for the change of condition. It is necessary, however, for this purpose, that the form should be carefully maintained during the exposure to the heat necessary to effect the vulcanization which leaves it in a normal state. A vulcanized solid sphere of 2 inches in diameter, when forced between two rollers inch apart was found to maintain its form uninjured. In fact, it is the exclusive property of vulcanized caoutchouc to be able to retain any form impressed upon it, and to return to that form on the removal of any disturbing force which has been brought to act upon it. Caoutchouc slightly expands and contracts in different temperatures; it is also capable of being condensed under pressure. A tube of 24 inches, impactly secured, was subjected to a force of 200 tons. The result was a compression amounting too;-great heat appeared to have been evolved, and the excessive elasticity of the substance caused a flywheel weighing five tons to recoil with an alarming violence. The evolution of heatf rom caoutchouc under condensation is a property possessed by it in common with air and the metals. It differs, however, from the latter in being able to exhibit cold by reaction. Mr Brockedon stated that he had raised the temperature of an ounce of water 2° in about 15 minutes by collecting the heat evolved by the extension of caoutchouc thread: he refers this effect to the change in specific gravity. He contends that this heat thus produced is not due to friction; because the same amount of friction is occasioned in the contraction as in the extension of the substance, and the result of this contraction is to reduce the caoutchouc thus acted upon to its original temperature. Among the latest applications of the elastic force of caoutchouc-the chief purport of Mr Brockedon's lecture-attention was directed: 1. To Mr E. Smith's patent application of tubes of vulcanized caoutchouc as torsion springs to roller blinds,-adjusted to the heaviest external blinds of houses, or the most delicate carriage blinds; and equally applicable to clocks and various machines as a motive power. 2. To the raising of weights (Mr Hodges' patent application).—Short lengths of caoutchouc (termed by him Vulcanized power-purchases) are successively drawn down from or lifted to a fixed bearing, and attached to any weight which it is required to raise; when a sufficient number of these powerpurchases are fixed to the weight, their combined elastic force lifts it from the ground. Thus ten purchases of the elastic strength each of 50 lb. raise 500 lb. Each purchase is six inches long and contains about 1 oz. of vulcanized caoutchouc. These ten purchases, if stretched to their limit of elasticity, not of their cohesive strength, will lift 650 lb. This power-the accumulation of elastic force-though it obey the common law of mechanical powers, differs enough to be distinguished as a new mechanical power. The same principle is applicable to relieve boats in tow from the strain they are subject to, and to easing the strain on ship's cables, especially where several boats are towing one vessel. 3. Applied as a projectile force. A number of power-purchases, attached to the barrel of a gun constructed to project harpoons, will exert a power if suddenly relieved proportioned to their aggregate forces. Similar contrivances have been made for projecting balls 200 yards or more: a charge of No. 4 shot can be thrown 120 yards. On the same principle a bow was contrived in which (reversing the usual form) the string alone was elastic; this bow throws a 30-inch arrow 170 yards. There were also exhibited adaptations of this material, for restraining furious horses,-for slinging horses whose limbs have been broken,—for enabling bed-ridden persons to assist themselves, for strengthening feeble joints, and many other new and valuable purposes. On Impressions of Rain-drops in Ancient and Modern Footprints of reptiles and birds have been observed on the surface of several ancient strata, accompanied by cracks resulting from the shrinkage of mud during desiccation, and it had been fairly inferred that the rocks bearing these marks must have been formed on a beach, between the level of high and low tide. It might therefore have been presumed that the same combination of circumstances would favour the preservation of impressions left by rain-drops, if any rain had fallen on the surface of the same strata, when in a state of mud or sand. Accordingly, memorials of rain have been met with, and Sir Charles Lyell exhibited specimens of fossil rain and hail-prints, collected by Mr Redfield of New York, from the New Red Sandstone of triassic age in New Jersey and others of still older date, obtained by Mr Richard Brown, from green slabs and sandstone of the Coal Measures of Cape Breton in Nova Scotia. Casts of rain-drops were first recognised in 1828 by Dr Buckland on the lower surfaces of slabs of quartzose sandstone, found by Mr Cunningham in the Storeton Hill quarries in Cheshire, where they are accompanied by shrinkage cracks, footprints of Cheirotherium, and ripple marks. Mr Redfield and Sir C. Lyell observed others at Newark in New Jersey in 1841, in red sandstone and shale; and still finer examples have been since met with at Pompton, in the same state, twenty-five miles from New York, by Mr Redfield. The Lecturer had also an opportunity of observing, in 1842, that a shower of rain had left numerous impressions on the mudflats exposed at low water, in estuaries communicating with the Bay of Fundy; and he afterwards obtained a collection of specimens of the hardened mud from Dr Webster of Kent *Abstract of a Lecture delivered in the Royal Institution, 4th April, 1851. ville, some of which are marked by the drops of a heavy but transient shower which fell on the 21st of July 1849. The average size of the hemispherical cavities is small, but some of them are no less than half an inch in diameter. Many of them are circular, but in some the longest diameter exceeds the shortest by 1th, or even 3d. They are surrounded by a small rim of mud, consisting of the matter which has been forcibly expelled from the pit by the falling drop; and this marginal rim sometimes projects as much above the plane of the stratum, as the bottom of the pit extends below it. In those impressions which have been made when the wind was blowing, and when the rain fell obliquely, the cavities are not only of an oval shape, but all deeper at one end than at the other. Footprints of birds, and the winding tubular tracks of annelids are seen on the same surface with the rain-prints. On splitting open slabs formed by numerous thin layers deposited by successive tides, impressions of previous showers are seen, and casts of the same, standing out in relief on the under surface of incumbent layers. The Lecturer next considered the nature of certain small protuberances, which might, on a cursory view, be mistaken for easts, which project from the upper surface of certain layers of mud, and are caused, some of them by dried bubbles of mud, and others by small particles of solid matter, covered with a film of mud. He also distinguished between the cavities produced by air-bubbles rising up through the mud, which give rise to cavities differing in shape from those formed by rain, as he has proved by several experiments. In illustration of the foot-tracks of quadrupeds, such as the musk-rat, the minc, the dog, and others, so common on the recent red sand of Kentville, on the borders of the Bay of Fundy in Nova Scotia, Sir C. Lyell exhibited a copy of a brick one foot square from Babylon, now in the British Museum, on which the track of a small animal of the Ichneumon tribe, apparently the Asiatic Mongoose, is distinctly seen. This brick has been sun-dried (not baked in a kiln), and must have been traversed by the creature, when the clay mixed with straw was still very soft. In the middle of the brick is an inscription in the Babylonian cuneiform character, which |