274 PROCEEDINGS OF THE AMERICAN ACADEMY. 



vided of course that none of the components in the gas to be analyzed is 

 soluble in water. As will be shown, the use of water greatly simplifies 

 the measurement of pressure, for an appreciable error in the height of 

 the column causes no appreciable effect on the result. Hence no cathe- 

 tometer is needed, and the simplest apparatus suffices. 



Although a marked tube (such as an inverted burette) may be used 

 for the attainment of constant volume, the measuring apparatus consists 

 preferably of a bulb of about 50 c.c. capacity with a single mark upon its 

 stem.* This bulb should have a capillary tube above with an internal 

 diameter not far from 1 mm.,t while below it is provided with a straight 

 tube about 20 cm. long and 7 mm. in diameter, with a sharp line etched 

 just below the bulb. To this tube is attached a levelling bulb by means 

 of a rubber tube about 150 cm. long.| The levelling bulb is conveniently 

 hung from a bent stick, which may be clamped to the iron ring-stand, 

 either above or below the table-top. Of course the bulb should be en- 

 closed in a water-jacket, conveniently made from an inverted broken 

 bottle, in order to insure constancy of temperature. The arrangement 

 is represented in the Figure. 



Id case the full absorption of 25 per cent is expected, it is conven- 

 ient, although not necessary, to begin with an excess of pressure. So 

 much gas is put into the apparatus for analysis that a pressure of about 

 a meter of water (measured by means of a wooden meter-rule) is neces- 

 sary to compress the gas exactly to the mark on the stem. About 10 

 per cent (5 c.c.) more gas than the amount needed to fill the space under 

 atmospheric pressure may be introduced for this jjurpose. The height 

 of this water-column is reduced to the mercury standard at 0° by multi- 

 plying it by about jg^ ; § andjthe reduced value is added to the barometric 



* Mr. W. N. Stull has suggested the use of a tube with several connected bulbs 

 witli as many marks in order to attain a wider range with loss ciiange of pressure. 

 This apparatus will soon be tested in this Laboratory. 



t Tiie rubber tube attached to the capillary should be very thick-walled and 

 have an exceedingly fine bore. It should be wired to the glass in the manner 

 shown in the diagram. 



t In order to prevent the admission of small air-bubbles into the measuring bulb 

 from tlie long rubber tube, a trap may be formed by bending tlie lower glass tube 

 through 100°, or else by making a loop in the rubber tube in the fashion shown in 

 the diagram. This precaution is very rarely necessary, however ; it is much better 

 to be certain in the first place that no such air-bubbles are present. 



§ The divisors corresponding to various temperatures of water column are as 



follows : — 



10° Divisor = lii.GO 20° Divisor = 13.62 



15° „ =13.61 25° „ =13.63 



