1908] BRIEFER ARTICLES 51 



the diameter of the bore, two gages were calibrated. It was found, 

 however, that while for high pressures, involving the measurement of short 

 lengths of tube, it would be necessary to make the correction for the dif- 

 ferences in diameter of the bore, this is not needed in dealing with small 

 pressures, of about one atmosphere, because in the length of tube used the 

 inequalities, which are very slight, balance one another. 



For use the gage is first filled with clean, dry mercury; then with 

 the lower end dipping into boiled water mercury is forced out into the water 

 by pressure with a rubber bulb upon the upper end until there remains 

 only the required amount, namely, enough to fill the gage half-way up the 

 bulb. When the force is removed, water replaces the mercury in the lower 

 end. The long arm is then filled with air, dried, by being drawn through 

 tubes containing calcium chlorid and phosphorus pentoxid, and is quickly 

 sealed with hot, melted shellac. Proper tests were of course applied to 

 prove that the shellac made a perfectly tight joint with the glass. Sealing 

 wax was' used at first, but it is apt to crack from the glass when the range 

 of temperature is great. The gage was then attached by means of stout- 

 walled rubber tubing to the stump of a plant whose top had been cut off 

 a-^cm above the gromid. The cutting was done under water to prevent 

 the entrance of air, which would rise in the gage later. The rubber tubing 

 was fastened on the stump with stretched rubber bands, or wire in case 

 of a stout stem, and to the manometer by wire and was wound with tire- 

 tape to prevent stretching under internal pressure, s 



Where the stem was larger than the gage, a glass sleeve was cemented 

 with sealing-wax outside the latter, to make it nearly the size of the stem. 

 The height of the mercury in the long arm was noted and when it had 

 reached its highest point the pressure was computed by Boyle's law.^ A 

 source of error in the use of the gage, not noted until too late for correction, 

 consists in the expansion of the mercury through rise in temperature. Sub- 

 sequent tests were made to ascertain its extreme possible amount, with a 

 result that under no circumstances could the error exceed 0.0134 atmospheres 

 or o . 2^^ per square inch, and In nearly all cases it was verj- much less than 

 this. 



The quantity of exudation was found at the same time for plants of ^ 

 same age and approximate size as those used in the exudation pressure 

 experiments. For this a piece of glass tubing was attached to the pl^t 

 and led into a graduated cylinder which contained a fihn of oH to prevent 

 evaporation. 



5 A fuller description of tHs gage with directions for ^j^f^^l^'^: 

 G.^>.oXG in a catak,gue ol apparatus now being published by the Bausch & I^mb 



Optical Company. 



