40 BLOOD METHOD. [CH. II 



a centimeter in length, but for an obvious result a single 

 leaf of Elodea or part of a Hottonia leaf seems to us 

 preferable. It is mounted in a large drop of blood which 

 is spread into a thick layer by supporting the cover-glass 

 on strips of paper. The preparation is now exposed to 

 sunlight for 3 or 4 minutes, or to bright diffused light 

 for a somewhat longer period. The oxygen given off 

 from the leaf brings back red arterial tint to the blood. 

 The effect is perhaps most easily seen under a low power 

 of the microscope, the red zone round the leaf giving the 

 impression of a source of light hidden behind the leaf, 

 producing in fact somewhat of a sunset effect. But it is 

 also clearly visible with the naked eye, especially if the 

 preparation is held above a sheet of white paper ; it is not 

 so plain if it lies on the paper. The venous blood has a 

 dull, almost grey purple appearance in contrast to the red 

 halo which surrounds and follows the curves of the leaf 

 The effect is quite clear and unmistakeable. A control 

 specimen should be placed in the dark so as to make sure 

 that the effect is not due to diffusion from the gas in the 

 intercellular spaces of the leaf Or the specimen which 

 has been illuminated may be darkened, for about 1 hour, 

 or until the red colour disappears, when the light effect 

 may once more be produced. 



According to Engelmann the most delicate method of 

 showing the evolution of the oxygen is by means of the 

 spectroscope, the spectrum of the blood changing as the 

 oxyhsemoglobin appears. 



To perform Hoppe-Seyler's* version of the experiment 

 1 Zeitschr.f. physiolog. Chemie, Bd. ii. p. 425, 1879. 



