68 



PLANT PHYSIOLOGY. 



but this is so slight as to be negligible for general work. (See Note 2 of 

 the Addenda.) Another would be the heating and expansion of the air 

 in sealing the tube, but this is very slight and almost entirely avoidable. 

 A method in some ways better is to use instead of this plain tube a 

 stop-cock tube (shown in Fig. 12) ; it is used precisely as is the plain tube, 

 except that water may be added at once to bring the liquid to the zero 

 mark of the scale, when the stop-cock is to be closed ; (any drop through 

 negative pressure will compensate itself.) The objection to the use of 

 these tubes, aside from the expense, is the considerable difficulty of mak- 

 ing them tight to pressures from within. For a method of testing their 

 tightness see page 41. Some of them will remain tight to an atmos- 





FIG. 12. FORMS OF PRESSURE-GAUGES, FOR MEASURING ROOT AND OTHER 



OSMOTIC PRESSURES. 

 One-fourth the true size. 



phere of pressure for several days, but then will leak. They are tightest 

 when very little of the stop-cock wax is used, and when the cocks are 

 thoroughly screwed into place. 



Boyle's (also called Mariotte's) law is this, that, the temperature being 

 the same, the volume of a gas varies inversely as the pressure upon it. 

 Thus if the volume of a gas becomes compressed to one-half its former 

 volume, the pressure has been doubled ; if compressed to f its former 

 volume, the pressure is f, i.e., half as much again as at the start. If the 

 liquid in the gauge above described rises so as to compress the volume 

 of air to f the volume it has at the start, the pressure must be f of what 

 it was at the start. But it was one atmosphere at the start, and hence 



