226 Dr. Alex. Hodgkinson on 



on by various reagents and solvents. " Thus, cyanine and 

 aniline blue dissolved in alcohol give very similar spectra, 

 dissolved in water totally different ones. The absorption 

 bands of oxyhaemoglobin disappear with reducing agents, 

 those of carmine, which are in a similar position, do 

 not ; the bands of braseline disappear when acetic acid is 

 added to the solution, those of fuchsine do not, &c." Such 

 facts as the above conclusively demonstrate the importance 

 of exercising caution in drawing inferences from the mere 

 spectroscopic appearance of bodies. With such an over- 

 whelming array of objections to this method of analysis it 

 might be supposed to be of little practical value. The 

 contrary is, however, the truth. " The study of the absorp- 

 tion spectra of various physiological fluids," remarks 

 MacMunn in his valuable work ' The Spectroscope in 

 Medicine,' "has taught us more than their ordinary chemical 

 analyses. More especially is this the case with blood ; for 

 not only has the spectroscope shown why arterial differs 

 from venous blood in colour, but it also enables us to detect 

 the minutest trace of blood ; and also to understand how 

 certain gases, &c, of a poisonous nature put an end to 

 animal life." The observations of Kundt, Vogel, and others, 

 showing the varying results obtained where no such varia- 

 tion was expected, merely point to the necessity of further 

 observation to explain the apparent exceptions, and of im- 

 provement in the method of research, that our observations 

 may be more reliable. Certainly no good can arise from 

 under-estimating, as several observers do, the importance of 

 the position of absorption bands in the spectrum. So far 

 as spectroscopy is concerned this must ever constitute a 

 fact of paramount importance, and I am convinced that 

 whatever tends to ensure a more accurate determination of 

 the relative position of absorption bands must tend to place 

 absorption spectroscopy, as an accurate method of research, 

 at a more measurable distance from radiation spectroscopy 



