152 Proceedings of the Royal Society of Edinburgh. [Sess. 
induce any change readily perceptible to the eye. The general trend of 
the experiment shown in the graph, however, is clear. It is probable, 
nevertheless, that the course of the reaction would be more truly indicated 
by greater steepness of the curve. 
1ST 
Fig. 2. — Effect of — — sodium nitrite on the different concentrations 
400 
of blood indicated against the curves. 
Ordinates , extinction coefficients ; Abscissae, time in minutes. 
Effect of Different Concentrations of Sodium Nitrite . — Although this is 
of the same order as the class of experiment just described, it is worthy 
of separate mention. In the previous case the concentration of sodium 
nitrite was kept constant ; in these experiments the concentration of blood 
was kept constant, or as constant as the measuring of small volumes of 
solutions allows. With the method of investigation adopted, higher con- 
centrations of sodium nitrite than centinormal could not be used owing 
to the rapidity of their action on the blood, unless by some means the 
action was delayed. Even with centinormal solutions the time of com- 
mencement of the reaction could not be observed. In the curve given 
(fig. 3) it was probably not before ten seconds from the time of com- 
mencement of the mixing. If this was the induction period, it is interesting 
to note that the two-hundredth normal solution took about seven times as 
long, and that the four-hundredth normal solution took about seven times 
as long as the two-hundredth normal solution. Once the reaction had 
