Living things are all about us. More than a million different kinds of plants and animals inhabit the earth. Some are our friends, others are our enemies. Some are very large and some are very small. Yet each is a distinct organism, and each has its own way of living.
You cannot learn the names of all the living things on the earth. Fortunately, there are groups of ani≠mals and groups of plants that greatly resemble each other. Because of this fact living things may be classified into large groups.
To study living things, it is necessary to sort them into groups. About a million and a half different kinds of plants and animals have already been studied, identi≠fied and named. In fact, for people who have not studied biology, the living world is a hopeless conglomeration of individual plants and animals.
X. Give the key words to the text. Give the summary.
Read and translate the following words:
cloud, droplet, tiny, rare, climate, to solidify, freezing, spherical, diameter, cubic meter, light, dense, investigator, puzzle, disappear, hollow, to turn out, predictable rate, evaporation, to buoy , affinity, to initiate, saturation, humidity, availability.
Fog is simply a cloud, composed, like any cloud, of tiny droplets of water or, in rare cases, of ice crystals, forming an ice fog. Ice fogs usually occur only in extremely cold climates, because the water droplets in a cloud are so tiny they do not solidify until the air temperature is far below freezing, generally 30 degrees below zero Celsius or lower.
The droplets of fog are nearly spherical; they vary in diameter between two and 50 microns and in concentration between 20 and 500 droplets per cubic centimeter of air. The transparency of a fog depends mainly on the concentration of droplets; the more droplets, the denser thefog. A wet sea fog may contain a gram of water per cubic meter; a very light fog may have as little as 0.02 gram of water per cubic meter.
Since water is 800 times denser than air, investigators were long puzzled as to why fogs didnít quickly disappear through fallout of the water particles to the ground. To explain the persistence of fogs many early investigators concluded that the droplets must be hollow (that is, bubbles). It turns out, however, that the droplets are fully liquid and do fall at the predictable rate, but in fog-creating conditions they either are buoyed up by rising air currents or are continually replaced by new droplets condensing from the water vapor in the air.
The atmosphere always contains some water vapor, supplied by evaporation from different bodiesof water, vegetation and other sources. The droplets condense on tiny particles of dust in the air called condensation nuclei. These are hygroscopic particles, which because of their affinity for water vapor initiate condensation at subsaturation humidities ó sometimes as low as 65 per cent. The nucleus on which the water condenses, which may be a soil particle or a grain of sea salt, a combustion product or cosmic dust, usually dissolves in the droplet. Because the saturation point is lower in solution that it is for pure water, the droplets of solution tend to condense more water vapor on them and grow in size.
Given suitable conditions of temperature and humidity, the density of a fog and its microphysical properties will depend on the availability of condensation nuclei and their nature. Fogs become particularly dense near certain industrial plants because of the high concentration of hydroscopic combustion particles in the air.