46 LECTURES TO SCIENCE TEACHERS. 



deeper down, and after a certain time the tipper portion of 

 the blood is seen of the scarlet colour, and the under portion 

 of the purple colour. If you then put the test-tube behind 

 a slit, such as I have shown you, and look at it through a 

 prism, you will see the two spectra simultaneously, as repre- 

 sented in the figure. 



To take another example, I have here a solution of per- 

 manganate of potash. If it is considerably diluted, and you 

 analyse the light transmitted through, you will see a broad 

 dark band in the spectrum. If you have it more diluted, you 

 obtain a spectrum highly characteristic, in which are seen five 

 dark bands in the green part of the spectrum. Those are 

 highly characteristic of the permanganates. There is just a 

 trace of a sixth band, which comes in when the solution is 

 stronger. These are alternations of transparency and opacity, 

 not that the fluid is perfectly transparent, but these inter- 

 vening spaces are really alternations of greater and less absorp- 

 tion. When the quantity present is sufficient, the whole of 

 this region is absorbed, and then the characteristics are lost, 

 because there are a great variety of purple substances which 

 would give a spectrum not very different. In examining a 

 substance you must dilute the solution to make sure of break- 

 ing up any such broad dark region, and then you see the 

 dark bands, if any, which are characteristic of the substance. 

 There are other red solutions of manganese which may be 

 obtained, and which agree with the permanganates in being 

 powerfully oxidising agents, and which long ago were con- 

 founded by chemists with the permanganates because they 

 have both- the purple colour, and are powerfully oxidising 

 agents. For example, if you rub up binoxide of manganese 

 with binoxalate of potash, you obtain one of these purple 

 coloured solutions, though it is not very permanent, which as 

 being a powerful oxidising agent and also of a purple colour, 

 was supposed to contain permanganic acid, but the spec- 

 trum instantly shows you it is nothing of the kind. These 

 two examples will suffice to show how valuable the prism is, 

 even without any other apparatus, as a means of discrimi- 

 nating between different bodies. 



The phenomena of the coloration of natural bodies is 

 best studied, as I said before, in coloured solutions ; but I now 

 pass on from that to the colours of natural bodies as commonly 

 presented to us. Let us take, for example, a very common 



