124 Hemoglobin Determination 
gas fixture in order that when solution is withdrawn from the 
bottom, gas rather than air will enter to replace it. Solutions thus 
prepared may keep for several weeks; but, as a precaution, it is 
advisable to make fresh 1 per cent solutions frequently; 2.e., 
every 2 or 3 weeks. It should be remembered that dilute hemo- 
globin solutions are less stable than concentrated solutions; and 
that hemoglobin solutions keep best in the cold and protected 
from light. The first indication of solution deterioration is a 
change in color from the characteristic cherry-red of carbon 
monoxide hemoglobin to a red with a brownish tinge, due to the 
formation of methemoglobin. 
Comparison in Colorimeters.—The. Duboseq or Kober colorim- 
eters have proven to be by far the most accurate and satis- 
factory instruments for this colorimetric work. Other colorimeters, 
however, may be used. The difficulties encountered are those 
inherent in all colorimetric work and in this connection reference 
to Kober’s (12) article may be made. The color of the carbon 
monoxide hemoglobin, because of the relatively low stimulus 
threshold for the eye, is admirably suited to colorimeter compari- 
son, slight differences being easily detected. We prefer to use 
the daylight from a north window. Satisfactory results are, 
however, obtainable with artificial light when “daylight glass”’ is 
used between the source of light and the solution. Considerable 
experimentation with light filters has failed to improve on the 
accuracy with which the comparison may be made. No diffi- 
culty should be experienced in making the readings check within | 
0.2 of a single division on the colorimeter scale. 
Color comparisons are most accurate when the unknown 
hemoglobin solution reading falls between 9 and 11 on the colo- 
rimeter scale. If the reading of the unknown falls below 8 or 
above 12, another sample should be taken and the dilution made 
such that the reading will fall within these limits. This is easily 
accomplished by varying the amounts of blood and ammonia 
solution, making the necessary correction in the colorimeter. 
2 cc. of solution is adequate for the Kober instrument and 5 ce. 
for the Duboseq. If a Duboseq or a Kober colorimeter is not 
available, the Hellige instrument may be used. The 1 per 
cent hemoglobin standard may be sealed with paraffin into 
the wedge and the wedge, when not in use, kept in the ice box 
