Apparatus and Methods 73 



plete absorption; during this time there may be an appreciable barometric 

 change. Practically all modern gas-analysis apparatus provides for this 

 change in temperature by adjusting the compensating pipette so as to 

 take care not only of the changes in temperature but likewise changes in 

 barometric pressure. Under these conditions it can be safely said that 

 within reasonable limits all changes in temperature and pressure are readily 

 compensated by the modern compensating pipette of the best forms of 

 gas-analysis apparatus. 



Tension of aqueous vapor. The varying percentages of water in sam- 

 ples of air and the differences in tension of the aqueous vapor above the 

 absorbing solutions make it necessary to insure that the tension of gases 

 before and after absorption remains exactly the same. This is best se- 

 cured in practically all modern gas-analysis apparatus by saturating the 

 gas with water-vapor both before and after absorption. A satisfactory 

 method for this is the placing of a few drops of water upon the surface 

 of the mercury which is ordinarily used as the liquid for inclosing and 

 measuring the sample. Under all conditions, therefore, the gas as meas- 

 ured is saturated with water-vapor at the temperature of the water-bath. 

 If in the compensating pipette both water-vapor and a slight excess of 

 water are present, then the tension of aqueous vapor is exactly the same 

 in the compensating pipette and in the measuring pipette. 



Contraction in volume as a measure of the oxygen absorbed. The most 

 difficult condition in gas-analysis apparatus is to make sure that the con- 

 traction in volume as measured is due only to the absorption of oxygen. 

 The usual procedure in measuring the gas is to read the top of the mer- 

 cury meniscus; obviously, this reading gives not only a measurement of 

 the gas to be analyzed, but of the water-vapor, and also of the liquid 

 water used to insure constancy in the tension of aqueous vapor. The ab- 

 sorption of the gas to be measured changes the level of the mercury, rais- 

 ing it materially; it is assumed that all of the liquid water adhering to the 

 walls of the tube is removed by the mercury as it rises, and that when the 

 mercury meniscus is again read the decrease in volume is due only to the 

 absorption of gas, the volume of liquid water present in the tube above 

 the mercury remaining essentially unchanged. The difficulties experi- 

 enced in proving this assumption have been practically insuperable, and 

 it has been necessary to resort to a reading of the water meniscus, which 

 is at best very unsatisfactory. Fortunately for purposes of compari- 

 son, when essentially the same gas as atmospheric air, for instance is 

 analyzed day after day, it is possible to arrange the conditions so as to 

 make the amount of water adhering to the walls of the tube practically 

 constant as the level of the mercury changes. Theoretically, therefore, 

 the best method for analyzing gases is to measure them absolutely dry 

 both before and after absorption in a perfectly dry and clean pipette over 

 absolutely dry and clean mercury. These conditions Dr. Krogh has 

 succeeded in securing in his new gas-analysis apparatus, which unfortuna- 

 tely has not as yet been described. 



