Clifton College Scientific Society. 85 



magnified before reaching the eye. The rays from each flame are 

 made to pass into the telescope (/) ; one set through the upper 

 uncovered half of the slit, the other by reflection from the sides 

 of the small prism (c) (which cannot be seen in the figure), through 

 the lower half, thus bringing the two spectra into the field of view 

 at once, so as to be able to make any comparison we like of the 

 lines. The small luminous gas flame (7i) is placed so as to illumi- 

 nate a fixed scale contained inside the tube {g) ; this is reflected 

 from the surface of the prism (a) into the telescope, and serves as 

 a means of measuring the position of the lines. {Roscoe) 



Now, let us observe the spectra of various substances. Every 

 different chemical element heated to luminosity gives off" a peculiar 

 light, which cannot in many instances be distinguished by the eye, 

 but can with the greatest ease by the spectroscope. For example, 

 when a potassium and rubidium salt are placed in a clear flame, 

 they both yield a purple colour, which appears of the same tint ; 

 but when placed in the spectroscope, we see that potassium gives 

 a violet and two red lines, while rubidium gives two violet, besides 

 a great number of green and red. In examining any substance, 

 it is best to heat the chloride in a colourless Bunseu's flame, as 

 this salt is generally the most volatile compound. Let us first 

 look at the spectrum of sodium. This is the most delicate of all. 

 It has been calculated that s;—-^ part of a milligramme, or i^^—^^, 

 part of a grain, of soda can easily be detected. Soda is always 

 present in the air, existing in almost every speck of dust, and 

 occurring most abundantly in sea-water and several mines in the 

 form of chloride. Whenever we clap our hands in front of a 

 Bunsen's flame, we observe a yellow colour in it. This is due to 

 sodium. There is only one line, Na a, characteristic of it, which 

 is coincident with Traunhofer's dark line D, and is remarkable 

 for its exactly defined form, and for its extraordinary degree of 

 brightness. (I shall speak further on about Fraunhofer's lines.) 

 If the temperature of the flame be very high, and the quantity of 

 the substance employed very large, traces of a continuous spectrum 

 are seen in the immediate neighbourhood of the line. 



Lithium, which was before considered to be a most rare and 

 scantily distributed substance, is now seen to exist in a great num- 

 ber of bodies, though in very small quantities ; being found in the 

 juice of plants, grapes, tea, coffee, milk, human blood, muscular 

 tissue, meteoric stones, the water of the Atlantic, manyrivers,mineral 

 waters, and the ashes of tobacco, besides in a great many minerals. 

 ^ part of a milligramme, or ^;^„ part of a grain, can easily be 

 detected. There are two lines which characterise the spectrum of 

 lithium ; the one a very weak yellow line, Lz /8, and the other 

 a bright red line, Li a. When the temperature is very high 

 a beautiful blue line is seen ; and it must here be noticed that the 



