This demonstration is of historical interest because it represents the way the Robert Boyle actually investigated the pressure-volume relationship for a gas. It also serves the important purpose of showing that Boyle’s law and all other laws derived from PV = nRT are accurate to three significant figures as long as conditions of pressure or temperature are not extreme.

Boyle J-tube apparatus (see Figure 1)

Small funnel to add mercury through top of apparatus

Plastic bottle of mercury

Small step stool to aid in ascending to the table top

Set up the apparatus (Figure 1) on a table before the class. Make absolutely sure that the stopcock at the bottom of the tube is closed!

It is helpful to recruit a member of the audience to write measurements on the blackboard. Get up on the table and pour enough mercury through a funnel into the J-tube to trap a sample of air. Remind the class that the gas temperature and amount will be constant throughout the demonstration. Read off the observed pressure and volume as shown in Figure 2. Repeat the mercury addition and the readings three more times. The last addition should fill the long side of the J-tube to around 80% capacity. The pressure will be in units of mmHg. The length in mm of the vertical space occupied by the trapped air will be proportional to the volume. The cross section area of the tube is uniform, so this is a good assumption. It is easiest to just proceed with the directly measured numbers in mm.

Figure 1. Boyle J-Tube apparatus

Figure 2. Reading gas pressure and volume with the J-Tube apparatus

Set up a simple chart on the blackboard to calculate the gas volume (actually mm) and pressure for each data point. Remember to add the ambient air pressure to the measured mercury column height. The pressure in Tucson is usually 700 mmHg give or take a little. The raw data will look something like that shown in the first table

The length of the column occupied by the air in the closed end of the J is the difference between the top of the closed end of the J and the mercury level in the left leg. For Experiment 1 this is 227 - 45 giving a column length of 182 mm. The pressure will be the difference between the two mercury legs plus the atmospheric pressure that day (assume 700 mmHg). For Experiment 1 this is (91 - 45) + 700 = 746 mmHg. The data for all four experiments are

Prior to the studies of Robert Boyle, few people would have believed that air was substantial enough to possess mechanical strength of any sort. Boyle was interested in the mechanical properties of air and the atmosphere. His early work involved the development and improvement of mechanical pumps for compressing or moving air, which are the forerunners of modern compressed air machinery, vacuum pumps, air brakes, shock absorbers, and other devices.

Boyle wanted to study the resistance of air to compression. Boyle devised an instrument requiring no moving parts. He fashioned a length of glass tubing, sealed at one end, into the shape of a "J". The idea was to trap a fixed amount of air in the closed end of the tube with liquid mercury. The volume of the trapped air could be found by measuring the diameter of the tube and the length of the air column. The pressure of the confined air is then the sum of the original atmospheric pressure measured with a mercury barometer plus the additional height of the mercury column.

Boyle could change the pressure at will by adding more mercury and thus study the relationship between pressure and the resulting volume of a fixed quantity of air. Examination of the PV data shows that as pressure increases, volume decreases. Of course Boyle also noticed this, but it was pointed out by several others examining his data that the product of pressure times volume was effectively constant.

There is no disposal required. After completing the demonstration, drain the mercury back into the plastic bottle and seal the bottle. The only conceivable hazard is a mercury spill resulting from leaving the stopcock at the bottom of the tube open or from tipping over the whole apparatus. If this happens, notify the preproom immediately!