44 LECTURES TO SCIENCE TEACHERS. 



another in their mode of production, I hardly like to em- 

 ploy it. 



You have seen in what manner you can readily, and 

 almost without any apparatus, observe a pure spectrum, and 

 how it is modified by the interposition of a coloured body ; 

 and I may just mention one or two instances of interesting 

 results which may be obtained in this manner. Sometimes 

 the mode in which a coloured medium attacks the different 

 parts of the spectrum is highly characteristic of the particular 

 iluid that you are employing. Here, for example, is one very 

 characteristic case the red colouring matter of blood. The 

 spectrum which that gives is represented in the upper part of 

 this figure [referred to]. In order to see the spectrum nothing 

 more is requisite than this : You take a slit of the roughest 

 description here is one made of wood and tinned iron 

 blackened, and the blood is conveniently held in a test-tube, 

 which you can hold in position by an elastic band. In order 

 to see the spectrum by transmission you have nothing more to 

 do than to hold this against a source of light and look at it. 

 If you use, not a wedge-shaped vessel, but a test-tube, you 

 cannot be sure of not passing over some of the most interesting 

 parts of the phenomena, unless you go step by step, and use 

 several different thicknesses, or, which comes to the same 

 thing, different degrees of dilution. For instance, when a 

 solution of blood is so highly coloured as this, a great part of 

 the spectrum is cut off, and it may be that you will see 

 nothing but a broad black band, whereas, if I had used a 

 weaker solution or a test-tube of smaller diameter, I should have 

 seen certain highly characteristic phenomena of absorption. 

 In order to see these, the solution must be so diluted that it 

 is little more than pink. Then you will see these highly 

 characteristic dark bands of absorption. I know of no sub- 

 stance which can be confounded with blood if you simply 

 take the spectrum of it in this manner, unless possibly an out- 

 of-the-way substance, turacine, the colouring matter found in 

 the red feathers of the wings of the touraco, a bird found at 

 the Cape of Good Hope. If you only looked at the spectrum 

 in one condition, it is possible that the two might be con- 

 founded, although hardly so ; but if you combine the obser- 

 vation of one of these peculiar spectra with the observation 

 of the effect of re-agents, you get a combination of characters 

 which is such that it is almost impossible to confound any 



