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Barium Saccharate

mercury, pressure and tube

BARIUM SACCHARATE (Formula, Ba0C, ).—Barium saccharate is sold as a white crystalline powder, soluble in water, and poisonous. It is sometimes employed as an alkali in development.

BAROMETER.—An instrument for estimating the weight or pressure of the atmosphere, invented by Torricelli in 1643. Torricelli discovered that if a tube closed at one end and free from moisture be filled with mercury and turned over in a dish containing mercury, the mercury in the tube will sink to a certain height (3o in.), leaving a perfect vacuum at the top. When the pressure of the air is increased the mercury rises, and falls on its decrease. Upon this principle barometers are constructed. By means of a float on the mercury and a wheel, the movements of the mercury are communicated to the pointer, or hand, on the visible dial.

The aneroid barometer is now largely used. The term "aneroid " is given to it because it is "without moisture." In an instrument of this kind the atmospheric pressure is held in equili brium by an elastic metal spring or tube. A metal box is freed from air and hermetically sealed. This box has a flexible side, the elasticity of which and the pressure of the air upon it keep each other in equilibrium. Upon this elastic side the short arm of a lever is pressed, while the longer

arm works an index point, as in the circular barometer. When the pressure increases the elastic box gives, and when the pressure diminishes it returns to its former place, and the index moves in the opposite direction. A curved tube is sometimes used, which coils and uncoils like a spring according to the pressure on it.

The uses of the barometer are various. It is employed to calculate the height of moun tains, for a barometer at sea level stands much higher than on a mountain top, because the amount of air is less, and consequently exercises less pressure upon the mercury.

In all barometric observations there are two essential corrections to be made, one for the capillarity or depression of the mercury in the tube, and the other for temperature. The following are the corrections for the capillarity or depression of the mercury in tubes of different diameters, as pure mercury always assumes a convex surface in a glass tube. These corrections are made according to the theory of Ivory.*