42 MEASUREMENTS 



by noting the effect on certain dyes or, more precisely, with an electrical 

 pH meter (Chapter 13). 



Pressure: The pressure exerted by a gas is expressed in a variety of 

 terms. The atmosphere exerts on a unit area of the earth's surface a 

 pressure which is equal to the mass of all the air vertically over that 

 area. Under a set of standard conditions, this pressure would be called 

 one atmosphere. Atmospheric pressure varies gready with temperature, 

 however, so some more easily measurable unit of pressure is desirable. 

 A column of mercury could be arranged so that it exerted the same 

 pressure as the standard atmosphere. Since mercury is so much more 

 dense than air, this column is only 760 mm high. In other words, a layer 

 of mercury 760 mm deep weighs the same as the whole thickness of the 

 atmosphere. If we used water, the column would be about 34 ft high. 

 In the laboratory, pressures usually are expressed in terms of the equiva- 

 lent column of mercury (mm Hg). The mass of such a column of mer- 

 cury depends on its cross-sectional area. A column of mercury 1 cm" in 

 cross section and 760 mm high weighs about one kilogram. This is 

 roughly equivalent to 14.7 lb/in". 



Even though most laboratory work designates pressures in mm Hg, the 

 international unit of pressure is the bar, which is equal to lO*' dynes/cm", 

 or 1.013 kg/cm^, or 0.987 atm. Weathermen express pressure in milli- 

 bars. 



Volumetric glassware 



It is possible to perform worthwhile biological experiments using noth- 

 ing more than ordinary glassware, and even those experiments employ- 

 ing the most elegant instruments are likely also to require volumetric 

 ware. This glassware is a special kind of laboratory equipment, designed 

 for measuring the volumes of liquids. Burettes, pipettes, volumetric flasks, 

 and graduated cylinders are the most commonly used pieces. 



A burette is a glass tube, graduated in milliliters, with a stopcock at 

 the lower end so that measured volumes of liquid can be drained off into 

 another container. Some burettes have special stopcocks and reservoirs 

 to make filling easier. When water or aqueous solutions are used in a 

 burette, the upper surface of the liquid forms a curve or meniscus. The 

 level of the liquid is measured by placing the bottom of this curve at the 

 graduation on the burette. For very precise reading some dark material 

 should be placed behind and below the meniscus so that the exact hot- 



