ABSORPTION OF LIGHT, ETC. 45 



other substance with the one which you have under your 

 hands. This becomes a mode of discrimination between sub- 

 stances of the utmost value to chemists, but which, strangely, 

 for a long time they altogether neglected, though, since the 

 researches of Kirchhoff and Bunsen, the chemical spectroscope 

 has become an instrument in the hands of almost every 

 chemist. 



I may mention one reaction with reference to the colouring 

 matter of blood which is interesting in itself, and will also 

 illustrate what I am saying. You know that the venous 

 and arterial blood differ from each other in colour. If you 

 look at the veins at the wrist you can see the redness of 

 the arterial blood in the arteries which happen to be near 

 enough to the surface, as contrasted with the deeper and darker 

 colour of the venous blood. This difference can be imitated 

 by introducing into a solution of blood a suitable deoxidising 

 agent, which will alter its colour. I have here, in the first 

 instance a solution of protosulphate of iron, and I have added 

 to that tartaric acid, which has the property of preventing the 

 precipitation of many metallic oxides. The colouring matter 

 of blood is immediately decomposed by acid, and therefore 

 you must take care not to introduce acid into the solution. I 

 have rendered this solution alkaline by ammonia without pre- 

 cipitating the iron. This is a strong reducing agent. It is 

 in small quantity almost colourless, and if a little of that is 

 introduced into the colouring matter of blood, which is 

 not decomposed in any reasonable time by ammonia, then 

 immediately the colour is changed into a purple one, and the 

 spectrum is changed in a remarkable manner, as represented 

 in the lower half of the diagram. In lieu of the two dark 

 bands, you have a single band occupying an intermediate 

 position. The fluid is purpler than before, and lets through 

 more blue light. If you have such a solution in a test-tube, 

 and shake it up with air so as to re-oxidise it, you get back 

 the original solution, and you may put it backwards and 

 forwards as often as you like. But I merely mention this as 

 illustrating what you get by using simply a prism without 

 any apparatus at all, and you can see the actual spectrum as 

 shown on the diagram. If a test-tube containing a solution 

 of blood deoxidised in this manner be allowed to stand for 

 some time, it absorbs oxygen from the air, the upper part 

 becomes oxidised, and this oxidation extends deeper and 



