46 



of analysis. For example, the exact position of the dark 

 absorption bands in the spectrum obtained fi'om blood is of 

 an important character ; it requires but little dispersion, and 

 it is necessary to use the Micrometer for measurements, as 

 well as suitable tests for proof. A solution of Oudhear in 

 dilute alcohol yields two very faint bands in much the same 

 place as those of fresh blood, but on adding ammonia one 

 band becomes very dark and distinct, and the lower band dis- 

 appears altogether. The addition of ammonia to a solution 

 of fresh blood produces no such changes. If ammonia is 

 added to a solution of cochineal, two absorption bands are 

 produced in much the same position as those of fresh blood, 

 and without care they might be (Confounded. 



There are many objects like the foregoing which require 

 careful and proper appliances ; there are also many others of a 

 much more characteristic kind. A striking spectrum is 

 produced by a very dilute solution of Permanganate of Potash^ 

 which gives five well marked absorptions bands at about equal 

 intervals in the green, and one or more in the blues. Aniline 

 Med, Madder, coloured crystals, and many other substances, 

 make easy and suitable objects for the Microscope, if the 

 solutions are not made too strong, so as to destroy the delicate 

 absorption bands. Sometimes these absorption bands are 

 broad and indistinct, but in other cases narrow and well 

 defined, and they are then much more characteristic. 



The best known object for testing fine definition is a pale 

 blue solution of Chloride of Cobalt in a concentrated solution 

 of Chloride of Calcium. If the two lines are seen in the 

 orange the definition must be very satisfactory. 



To show the effects produced by means of Spectrum Analysis 

 under the Microscope I have prepared very weak solutions of 

 the following substances, viz. : — Permanganate of Potash, 

 Aneline Eed, Madder, and Cochineal ; also fi-esh blood largely 

 diluted. It must, however, be understood that a casual ex- 

 hibition of these spectra is one thing and a scientific enquiry 

 is another, for every part of the spectrum differs from the 

 adjacent parts in refrangibility, and delicate bands can only 

 be brought out by accurately focussing their own parts. It is, 

 therefore, necessary to close the shutters in the Spectroscopes 

 over the slit on the remainder of the object, by means of the 

 two levers provided for the purpose, leaving only that portion 

 of the spectrum visible which is under examination. 



It will therefore appear clear that time will not admit of 

 entering into these details, as the object intended this evening 

 was more to explain and illustrate the principle adopted for 

 ascertaining the characteristic spectra produced from the 

 luminous vapours given off by the combustion of the following 



