222 I!O\V TO WORK 



B is the spectrum of a solution of logwood in water, to which 

 bicarbonate of ammonia has been added, and C is the same in 

 the case of Brazil wood ; and the difference between the two 

 is shown by the different position of a single well-defined absorp- 

 tion band. 



D is the spectrum of fresh Wood. 



E is the spectrum of alkanet root in alum with a little alcohol. 



F is the spectrum of deoxidised ammoniacal haematine. 



These three show very well how closely related spectra may be 

 easily distinguished by the different position and relative width and 

 darkness of the bands. 



Many coloured substances give spectra which do not enable 

 us to decide with confidence what they are. Perhaps some half 

 dozen substances may be known which would give the same 

 result, and this spectrum may only serve to indicate to what 

 group the colour belongs ; but even then, supposing it be a solution, 

 the addition of some reagent may at once show which particular 

 substance is present. For example solutions of Magenta and Brazil 

 wood both give a single well-defined absorption band in the same 

 position, in the upper part of the green, but ammonia produces 

 no change in Magenta, and great change in the Brazil wood. 

 Sulphate of soda then immediately makes the Magenta colourless, 

 but does not change the Brazil wood except by gradual fading and 

 decomposition. It is, however, extremely difficult to distinguish 

 some substances, especially when mixed with other colours ; but still 

 by suitable methods many can be recognised without difficulty 

 under most unpromising conditions. This, however, is more a 

 question for a treatise on qualitative analysis by means of the 

 spectrum microscope, than for one on the instrument itself and the 

 manner of using it. 



It must not be thought that an indefinite quantity of any 

 substance will give a characteristic result. If too little is present, 

 nothing definite can be seen ; and, if there is too much, the most 

 characteristic parts of the spectrum may be entirely obscured. For 

 example fresh blood gives two remarkably well-defined absorption 

 bands in the upper part of the green but, if too little is present, 

 these bands are very faint, and, if too much is present, all the light is 

 absorbed, except the red and orange, so that the bands cannot be 

 seen at all. In every case a certain amount of colour gives the best 

 result ; and though at first this may appear difficult to arrange, yet 

 after a little experience there is really no difficulty, especially if we 

 make use of such small cells as are shown in fig. 322. These are 

 cut from barometer tubes ; and I find that the most convenient 



