274 



PKOCEEDINGS OF THE AMERICAN ACADEMY. 



For solids, the stem D of the pipette may be made of wider tubing, 

 closed at the bottom with a perforated stopper. A small tube bent 



upwards may be inserted in this per- 

 foration, if especial precaution against 

 incoming air is desired. 



An explosion-pipette could be made 

 of similar apparatus, with the addition 

 of a stopcock just below the bulb A and 

 the usual conducting wires. 



The pipette for fuming acid might be 

 made with a ground-glass joint instead 

 of a stopper to connect bulb with bottle. 

 In that case the bottle should be pro- 

 vided with a suitable side tube on the 

 neck, bent upwards. 



The method of using these pipettes 

 will be understood without difficulty by 

 any one familiar with the Hempel 

 apparatus. 



II. Measuring Apparatus. 



The most serious cause of error in 

 Hempel's ordinary apparatus is due to 

 the possible change of temperature. 

 This is considerably greater than the 

 probable error in reading ; for a single 

 degree Celsius causes an error of 0.5 

 per cent of the total volume of gas 

 measured under ordinary conditions, 

 while the volume is easily read within 

 0.05 per cent. Hence, unless much 

 greater care than usual is taken to pre- 

 serve constant temperature, the reading 

 of the volume is unnecessarily precise. 

 But Hempel's ingenious arrangements 

 for maintaining constant conditions in a 100 c.c. burette are so large as 

 to be inconvenient for students' use in cramped quarters. 



For these reasons I have often used somewhat smaller volumes, which 

 may be surrounded with an envelope of water without producing thereby 

 an unwieldy combination. An ordinary 50 c. c. burette, inverted and pro- 



FlGURE 1. 



