256 LECTURES TO SCIENCE TEACHERS. 



any tolerable degree of exactitude to what the changes were 

 due. Here I have a little blood mixed with water in order 

 to render it transparent, as otherwise you would be unable 

 to see its spectrum. I have here a small spectroscope which 

 I will hand round, and will ask you, after directing the 

 spectroscope to the sky, to place the two test-tubes in 

 succession in front of its slit. First look at one spectrum 

 and then at the other ; one is arterial blood, and the other 

 is blood which I have rendered venous, that is to say, from 

 which I have abstracted the oxygen by adding a little of 

 this solution. The spectrum of the blood was first investi- 

 gated by Professor Stokes, and he imitated artificially the 

 changes which go on in the tissues by adding to the blood 

 a little of a fluid which will gradually abstract oxygen. 

 This fluid consists of a little protosulphate of iron, to which 

 some ammonia and tartaric acid have been added. When 

 I add a little of this to the blood, it takes away the 

 oxygen readily, and produces a change resembling that 

 which takes place when blood becomes venous. It does 

 not produce exactly the same changes in the blood that the 

 tissues do ; for although it abstracts the oxygen, it gives 

 back no carbonic acid. You will find that in one spectrum 

 you have two black bands, one in the orange, close to the 

 solar line D, and the other at the beginning of the green ; 

 this is arterial blood. In the venous blood you find that 

 these two bands have disappeared, and in their place is one 

 band broader than either, which fills up the spaces between 

 the places they would have occupied. If you were now to 

 close the test-tube containing the venous bood with your 

 finger, and shake it vigorously for a minute or two, so as 

 thoroughly to mix the air and the blood (as I do with 

 another specimen), and again examine the spectrum, you 

 would find that the single band of venous blood had dis- 

 appeared, and that the two bands of arterial blood had 

 again made their appearance. 



Now here I have imitated the changes which blood 

 undergoes in passing through the body. When I added 

 Stokes's fluid to it, I imitated the change it underwent in 

 the capillaries ; and when I shook it up again with air I 

 imitated the changes that it undergoes in the lungs. 



There are certain other peculiarities connected with this 

 haemoglobin. It forms, as I have told you, a combination 



