and that in two experiments 20 c.c. and 45 c.c. respectively of a liquid re-agent were employed. The true volumes calculated from the table would be as 19·66 to 44·46.
If the temperature remained constant throughout the above series of experiments, and if the temperature selected were 4° C., the weights of water found, taken in grams, give the volumes in cubic centimetres, for one gram of water at 4° C. has a volume of one cubic centimetre. If the temperature at which the experiments were made was other than 4° C., and if great accuracy be desired, a table of densities must be consulted, with the help of which the volume of any weight of water at a known temperature can be readily calculated.
Pipettes which are to be used as measuring instruments should also have the relation one to another of the volumes of liquid which they deliver determined, and also the proportions these bear to the values found for the divisions of the burettes in conjunction with which they will be employed.
To Calibrate Tubes for Measuring Gases.—Prepare a small glass tube sealed at one end and ground at the other to a plate of glass. The tube should hold about as much mercury as will fill 10 mm. divisions of the graduated tube. Fill this tube with mercury, removing all bubbles of air that adhere to the sides by closing the open end of the tube with the thumb, and washing them away with a large air-bubble left for the purpose. If any persistently remain, remove them by means of a fine piece of bone or wood. Then completely fill the tube with mercury, removing any bubbles that may be introduced in the operation, and remove the excess of mercury by placing the ground-glass plate on the mouth of the tube, and pressing it so as to force out all excess of mercury between the two surfaces. Clean the outside of the tube, and place it on a small stand (this may be a small wide-mouthed glass bottle), with which it has been previously weighed when empty, and re-weigh. Repeat this operation several times. From the mean of the results, which should differ one from another but very slightly, the capacity of the tube can be calculated.
The purest mercury obtainable should be used. Since the density of pure mercury at 0° C. is 13·596, the weight of mercury required to fill the tube at 0° C., taken in grams, when divided by 13·596, will give the capacity of the tube at 0° C. in cubic centimetres. If the experiment be not made at 0° C., and if a very exact determination of the capacity of the tube be required, the density of mercury must be corrected for expansion or contraction.
Having now a vessel of known capacity, it can be employed for ascertaining the capacities of the divisions of a graduated tube in the following manner:—The graduated tube is fixed perpendicularly, mouth upwards, in a secure position. The small tube of known capacity is filled with mercury as previously described, and its contents are transferred to the divided tube. The number of divisions which the known volume of mercury occupies is noted after all air-bubbles have been removed. This process is repeated until the divided tube is filled. A table of results is prepared, showing the number of divisions occupied by each known volume of mercury introduced.
In subsequently using the tube the volumes of the gases measured in it must be ascertained from the table of values thus prepared.
In observing the level of the mercury, unless a cathetometer is available, a slip of mirror should be held behind the mercury close to the tube, in such a position that the pupil which is visible on the looking-glass is divided into two parts by the surface of the mercury.
A correction must be introduced for the error caused by the meniscus of the mercury. As the closed end of the tube was downwards when each measured volume of mercury was introduced, and as the surface of mercury is convex, the volume of mercury in the tube when it is filled to any division l ([Fig. 41]) is represented by A of 1. But in subsequently measuring a gas over mercury in the same tube, when the mercury stands at the same division l, the volume of the gas will be as represented by B of 2, which is evidently somewhat greater than A. This will be seen still more clearly in 3, where a represents the boundary of the mercury, and b the boundary of the air, when the tube is filled to the mark l with mercury or a gas over mercury respectively.
