適合初中的英語美文摘抄
通過美文的賞析,陶冶學生的思想情操,開闊視野,提高對優美散文的鑑賞能力,提高人文素養;同時培養學生的認知水平、情感態度、文化意識,激發學習英語的興趣,爲他們的跨文化交際能力和終身學習英語的能力打下良好的基礎。本文是適合初中的英語美文，希望對大家有幫助!
Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plants acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of water in plants have been based on root pressure, a push on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures.
If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask: how does it get there? According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion (the attraction between water molecules) are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This cohesive strength permits columns of water to be pulled to great heights without being broken.
內聚壓力理論
大氣壓能夠支持10米高的水柱，但植物可將水送得更高。美洲紅杉就能把水泵到地面以上100多米高的樹頂。直到19世紀末，水在樹木和其它高大植物中的輸送還是一個謎。一些植物學家假定植物中的活細胞充當了水泵的角色。但許多實驗表明細胞都已死亡的植物莖幹仍能將水輸送到相當可觀的高度。對於植物中輸送水的其它解釋都基於根壓--植物底端的根對水的推動。但根壓完全不足以將水推到樹頂。
況且，最高樹木中的松柏只有很低的根壓。如果水不是被泵到高樹的樹頂，也不是被推到樹頂，那麼我們會問：它是怎樣到達樹頂的呢?根據目前爲人們所接受的內聚壓力的理論，水是被拉到上面去的。一株植物中作用於一個正在升高的水柱之上的拉力來自該植物頂部水的蒸發。
由於水從葉子表面喪失，一個負壓力，或張力就得以產生。蒸發出去的水被植物裏流動的水代替。這些水形成
水柱從植物頂端一直延伸到根部。在任何水樣中造成表面張力的力支持着這些不斷的水柱。
當水被限制在內徑很小的管道中時，內聚壓力(水分子之間的相互吸引力)是如此之大以致一支水柱的強度相當於一根直徑相同的鋼絲的強度。
這種內聚壓力使得水柱被拉到非常高的地方而不會斷裂。
適合初中的英語美文：The Salinity of Ocean WatersIf the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation--- conversion of liquid water to water vapor. In this manner the salinity is increased, since the salts stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind.
The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.
Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the formation and melting of sea ice. When sea water is frozen, the dissolved materials are left behind. In this manner, sea water directly materials are left behind. In this manner, sea water directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.
In the Weddell Sea Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.
海水鹽度如果我們分析海水的鹽度，會發現地區間只有輕微的變化，然而有些小的變化是重要的。
導致海洋的鹽度變化的基本過程有三個，其中之一是通過蒸發的方式即把液態水轉化爲水蒸氣來減少海洋中的水分。這樣由於鹽留了下來，所以鹽度增大。
當然，如果這種方式走向極端，將會餘下白色的鹽晶體。與蒸發相反的是降水，如降雨，由此水被加入海中，海水被稀釋，從而鹽度降低。
這種情形會發生在大量降雨的地區，或江河入海岸處。因此，鹽度通過蒸發減少水分而上升或通過降水或徑流增加淡水成分而下降。一般來說，在陽光很強烈的熱帶地區，海水的鹽度略高於世界上其它沒有熱帶那樣多的蒸發的地區。同理，在江河稀釋海水的海岸地帶，海水鹽度略低於其它海區。
第三個可以變更鹽度的過程與海洋中冰的形成和融化有關。海水凍結時，溶於其中的物質被留了下來。這樣，在新形成的海水冰面的正下方的海水比在冰形成之前有更高的鹽度。當然，當冰融化的時候，會降低周圍水中的鹽度。
在南極洲邊緣的威德爾海中，結冰過程增加低溫海水的鹽度，從而形成了濃度最大的海水。這些大密度的海水下沉，可以在世界海洋的深水域發現。
適合初中的英語美文：TopazTopaz is a hard, transparent mineral. It is a compound of aluminum, silica, and fluorine. Gem topaz is valuable. Jewelers call this variety of the stone “precious topaz”. The best-known precious topaz gems range in color from rich yellow to light brown or pinkish red. Topaz is one of the hardest gem minerals. In the mineral table of hardness, it has a rating of 8, which means that a knife cannot cut it, and that topaz will scratch quartz.
The golden variety of precious topaz is quite uncommon. Most of the world’s topaz is white or blue. The white and blue crystals of topaz are large, often weighing thousands of carats. For this reason, the value of topaz does not depend so much on its size as it does with diamonds and many other precious stones, where the value increases about four times with each doubling of weight. The value of a topaz is largely determined by its quality. But color is also important: blue topaz, for instance, is often irradiated to deepen and improve its color.
Blue topaz is often sold as aquamarine and a variety of brown quartz is widely sold as topaz. The quartz is much less brilliant and more plentiful than true topaz. Most of it is variety of amethyst: that heat has turned brown.
黃水晶
黃水晶是一種堅硬、透明的礦物質。它是鋁、硅和氟的化合物。黃水晶寶石價值不菲。珠寶商把這種石頭稱爲"黃玉"。
最出名的黃玉有各種顏色如深黃色、淡棕色、淺紅色等。黃水晶是最堅硬的寶石礦中的一種。在礦石硬度表上，它的硬度爲8，這表明刀子不能割開它而它可在石英上劃痕。金黃色的黃玉品種非常罕見。世界上大多數的黃水晶是白色或藍色的。這些白色或藍色的黃水晶晶體很大，常常有數千克拉重。
由於這個原因，黃水晶的價值不像鑽石和許多其它寶石那樣主要依賴於其大小，重量翻一番價值即上升約四倍。黃水晶的價值很大程度上取決於其品質，但顏色也很重要。舉例來說，藍色的黃水晶常需放射處理以加深和改善其顏色。
藍色的黃水晶常被作爲海藍寶石出售，許多種棕色石英被當作黃水晶廣爲販賣。石英光亮度遠小於黃水晶，礦藏儲量也遠較黃水晶豐富。大多數石英是一種紫水晶，高溫使其變爲棕色。
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