Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/134

114 boiling water in the jar, and a small quantity of boiling water is also placed in the globe. The tube is then fitted in, and the spirit lamp is lighted under the globe. The steam generated expels the air from the globe, and it bubbles up through the jar. When the bubbles of air cease to appear almost the whole of the air will have been ejected, and on withdrawing the lamp the steam in the globe condenses, creating a vacuum, to fill up which the infused coffee rushes up through the metal tube, being at the same time filtered by the accumulated coffee grounds around the perforated disc. An error of very frequent occurrence in the preparation of coffee, which results probably from the habit of tea-making, consists in using too little coffee. For a pint of the infusion from an ounce to an ounce and a half of coffee ought to be used. According to the experiments of Aubert a cup of coffee made from a Prussian loth (⋅587 oz.) contains from 1⋅5 to 1⋅9 grains of caffeine. Coffee belongs to the medicinal or auxiliary class of food substances, being solely valuable for its stimulant effect upon the nervous and vascular system. It produces a feeling of buoyancy and exhilaration comparable to a certain stage of alcoholic intoxication, but which does not end in depression or collapse. It increases the frequency of the pulse, lightens the sensation of fatigue, and it sustains the strength under prolonged and severe muscular exertion. The value of its hot infusion under the rigours of Arctic cold has been demonstrated in the experience of all Arctic explorers, and it is scarcely less useful in tropical regions, where it beneficially stimulates the action of the skin. It has been affirmed that coffee and other substances containing the alkaloid caffeine have an influence in retarding the waste of tissue in the human frame, but careful and extended observation has demonstrated that they have no such effect. Although by microscopic, physical, and chemical tests the purity of coffee can be determined with perfect certainty, yet ground coffee is subjected to many and extensive adulterations. Chief among the adulterant substances, if it can be so called, is chicory root; but it occupies a peculiar position, since very many people on the Continent as well as in Great Britain deliberately prefer a mixture of chicory with coffee to pure coffee. Chicory is indeed destitute of the stimulant alkaloid and essential oil for which coffee is valued; but the facts that it has stood the test of prolonged and extended use, and that its infusion is, in some localities, used alone, indicate that it performs some useful function in connection with coffee, as used at least by Western communities. For one thing, it yields a copious amount of soluble matter in infusion with hot water, and thus gives a specious appearance of strength and substance to what may be really only a very weak preparation of coffee. The mixture of chicory with coffee is easily detected by the microscope, the structure of both, which they retain after torrefaction, being very characteristic and distinct. The granules of coffee, moreover, remain hard and angular when mixed with water, to which they communicate but little colour; chicory, on the other hand, swelling up and softening, yields a deep brown colour to water in which it is thrown. The specific gravity of an infusion of chicory is also much higher than that of coffee. Among the numerous other substances used to adulterate coffee are roasted and ground roots of the dandelion, carrot, parsnip, and beet; beans, lupins, and other leguminous seeds; wheat, rice, and various cereal grains; the seeds of the broom, fenugreek, and iris; acorns; and &ldquo;negro coffee,&rdquo; the seeds of Cassia occidentalis. These with many more similar substances have not only been used as adulterants, but under various high-sounding names several of them have been introduced as substitutes for coffee; but they have neither merited nor obtained any success, and their sole effect has been to bring coffee into undeserved disrepute with the public. The leaves of the coffee tree contain caffeine in larger proportion than the seeds themselves, and their use as a substitute for tea has frequently been suggested. The leaves are actually so used in Sumatra, but being destitute of any attractive aroma such as is possessed by both tea and coffee, the infusion is not palatable. It is, moreover, not practicable to obtain both seeds and leaves from the same plant, and as the commercial demand is for the seed alone, no consideration either of profit or of any dietetic or economic advantage is likely to lead to the growth of coffee trees on account of their leaves.  COFFER-DAMS have from very early times been employed as useful, and in some cases indispensable, structures in executing works of marine and river engineering. By excluding the water from the area they enclose, the work can be carried on within them with nearly the same ease as on dry land. Whether used on a small or a large scale whether as low-tide dams of clay or concrete of only a few feet in height, or as high-water dams of timber and puddle formed to resist the waves of the sea, they are in every sense structures of great importance in the practice of hydraulic engineering. Tide-dams are chiefly used in laying the foundations of piers or other works that must be founded under low-water level. They are generally made of clay and planking, and are only carried to the height of about 3 feet above low-water. The water being pumped out during the last of the ebb tide affords one or two hours work at low-water, the dam being submerged on the rise of the tide. In such dams a sluice should be introduced, which when open may allow the water to escape with the falling tide and so save pumping. Such tide-dams when exposed to a considerable wash of sea may advantageously be made of cement rabble masonry, of the application of which to coffer-dams the earliest account we know is that stated in Stevenson s Account of the Bell-Rock Lighthouse (p. 230), where he successfully employed that method of construction in 1808 in excavating the foundation of that work. When it is required to sink the foundation some feet into sand and gravel, a convenient expedient is the portable dam proposed by Mr Thomas Stevenson described in the Trans, of the lioy. Scot. Society of Arts, 1848, to which reference is made. The feature in this tide-dam is the use of double framed walings to support and direct the driving of the sheet piles, and its advantages are its cheapness, its portability, and its ready adaptation to a sloping or even very irregular bottom. But when it is necessary entirely to exclude the water from large areas, as, for example, in dock-works, it is necessary to adopt coffer-dams of varied construction suited to the circumstances of each case, and as these protecting coffer-dam works, notwithstanding their temporary nature, demand much of the engineer s skill in their design and construction, we propose to notice some of the different modes of construction that have been adopted in such cases to suit the varying sub-soil and other features of different works. It may here be mentioned that, particularly in bridge building, caissons were employed in early times instead of coffer-dams, but they are now entirely out of use. The caisson was a flat-bottomed barge constructed of strongly framed timber-work, in which the under courses forming the foundation of the piers of a bridge, for example, were built at any convenient spot near the banks of the river. The caisson was then floated to the site of the pier, the bed of the river having previously been dredged so as to present a comparatively level and smooth surface. On the bed so prepared the caisson was sunk by admitting the water gradually by means of a valve provided for that 