276. =Thermometers.=--It is the expansion of liquids by heat that in the thermometer gives us the measure of temperature. The liquid metal mercury is commonly used for this purpose, and answers well except in the extreme cold of the arctic regions. There, as mercury becomes solid at 39 degrees below zero, it is necessary to use a thermometer with alcohol in it, as this fluid can not be frozen by any degree of cold. The operation of the thermometer is simply this: Heat expands the fluid in the bulb, and the only way in which it can occupy more space as it expands is by rising in the tube. The abstraction of heat, on the other hand, causes contraction, and of course a proportionate falling of the fluid. 277. =Fahrenheit's Thermometer.=--The thermometer was invented in the beginning of the seventeenth century, but it is not decided who was the inventor. There may have been in this case, as in others, more inventors than one, the same ideas having, perhaps, entered several inquiring minds at the same time. Various fluids were used by different persons. Sir Isaac Newton used linseed oil. Fahrenheit, a native of Hamburg, who flourished in the first part of the last century, was the first to use mercury. Though various propositions were made by Newton and others in regard to the measurement of heat by thermometers, no thermometric scale seems to have met with general reception till that of Fahrenheit's, which was put forth about 1720. The plan of it is this: His zero is the point at which the mercury stood in the coldest freezing mixture that he could make; and he supposed that this was the greatest possible degree of cold, as it was the greatest that he knew. He next found the point at which the mercury stood in melting ice. This he called the freezing point, because the temperature is the same in water passing into the solid from the fluid state as in water passing into the fluid state from the solid. In other words, this point in the scale marks the transition line between the two states. From this point Fahrenheit marked off 32 equal spaces or degrees down to zero. He now found the point at which the mercury stands in boiling water, and called this the boiling point. Marking off the space on the scale between this, and the freezing point in the same manner, there are 180 degrees--that is, the boiling point is 212 degrees above zero. The degrees above zero are commonly designated by the mark +, plus; and those below by the mark -, minus. Thus, +32° signifies 32 degrees above zero, and -32° signifies 32 degrees below. [Illustration: Fig. 195.]