cury is forced up the tube. The top of this column serves as an index. We know from Boyle's law that when the volume of the air has diminished one half, the pressure is doubled. The downward pressure of the mercury column makes up a part of this pressure; and the pressure acting on the surface of the mercury in the basin is greater than that indicated by the compression of the air in the tube, by the pressure due to the mercury column. For many purposes the manometer tube may be made very short, and the pressure of the mercury column that rises in it may be neglected.
125. Aneroids.—The aneroid is an instrument used to determine ordinary atmospheric pressures. On account both of its delicacy and its easy transportability, it is often used instead of the barometer. It consists of a metallic box, the cover of which is made of thin sheet metal corrugated in circular grooves. The air is partially exhausted from the box, and it is then sealed. Any change in the pressure of the atmosphere causes the corrugated top to move. This motion is very slight, but is made perceptible, either by a combination of levers, which amplifies it, or by an arm rigidly fixed on the top, the motion of which is observed by a microscope. The indications of. the aneroid are compared with those of a standard mercurial barometer, and an empirical scale is thus made, by means of which the aneroid may be used to determine pressures directly.
126. Limitations to the Accuracy of Boyle's Law.—In all the previous discussions we have dealt with gases as if they obeyed Boyle's law with absolute exactness. This, however, is not the case. In the first place, some gases at ordinary temperatures can be liquefied by pressure. As these gases approach more nearly the point of liquefaction, the product of the volume and pressure becomes less than it ought to be in accordance with Boyle's law.
Secondly, those gases which cannot be liquefied at ordinary temperatures by any pressure, however great, show a different departure from the law. For every gas, except hydrogen, there is a minimum value of the product . At ordinary temperatures and small pres-
