ABSORPTION 159 



upper end is at once sealed by a drop of heated shellac applied from a knife 

 point. The gauge, still standing in the water, should now be put aside for 

 some hours to allow the shellac to harden thoroughly. The short arm is 

 then attached to the plant or other object by a pressure-tight joint (described 

 in Part III) in the usual way. The pressure developed is of course calcu- 

 lated exactly by BOYLE'S Law. In exact work correction is not to be made for 

 the capillary depression of the mercury, but must be made for the weight of 

 the mercury column at the close. If at the close of the experiment the pres- 

 sure is released gradually, the gauges may be used repeatedly without 

 refilling; the shellac may be removed by alcohol. The drying of the air is 

 a refinement which may be omitted in elementary or other ordinary work, 

 since the error from moisture, especially if the gauge is filled in a dry room, 

 is ordinarily negligible. Third, though useful only for some demonstration 

 and elementary uses, there is the simplest possible form, a straight closed 

 tube in which the rising liquid compresses the air, the pressure being calcu- 

 lated by BOYLE'S Law. The tube should approximate in external diameter 

 to the stem, to which it is attached by the usual pressure-tight joint, but its 

 bore should be rather small so as not to require much liquid to fill it. It 

 should be about 10 cm. long and drawn out at one end to a conical tip end- 

 ing in a very slender open point which is sealed (with precaution against 

 heating the air in the tube) when the liquid commences to rise. A milli- 

 meter scale should be attached, and in making the calculations the conical 

 end is treated as a cone, whose capacity is one-third that of a cylinder of 

 the same height and base. A more convenient form of this gauge would 

 be one with a glass stop-cock at the upper end, but it is almost impossible 

 to make such stop-cocks tight against prolonged internal pressure. Such 

 gauges are liable to errors from (a) vapor-tension, (&) capillarity, (c) solu- 

 tion of some of the air in the water under increased pressure, and the escape 

 of some air from the ducts into the tube. The three former may be approxi- 

 mately determined and compensated from the correction tables in Part III, 

 but as the errors balance one another more or less, for demonstration or 

 qualitative work they may be ignored. 



ERRONEOUS USES OF PRESSURE-GAUGES. In a number of the current 

 text-books, and in some more special works which should know better, 

 open gauges of considerable bore are recommended, and these commonly 

 require very considerable quantities of water to push the recording column 

 of mercury to any considerable height. Now it frequently happens, as 

 Miss ECKERSON'S table, earlier cited, will show, that the quantity of water 

 exuded is very small, though its pressure is high; in such a case there would 

 only be water enough to push the mercury column a short distance, which 

 would be interpreted, but wholly erroneously, as meaning a low pressure. 

 Correct pressures can be recorded only by the use of gauges requiring quanti- 

 ties of water so small that full pressures can be registered by inappreciable 

 amounts, as is the case with the manometer above described. 



The student should now inform himself as to the quantities 

 of known root -pressures and exudation. 



