98 CHEMICAL DISCOVERY AND INVENTION 



as copper and zinc may be deposited one after the other from 

 the same solution. 



The current may also be employed for many experiments in 

 which the oxidising effect at the anode, or the reducing effect at 

 the cathode may be turned to advantage. This method has 

 been employed chiefly in connection with the study of organic 

 compounds. The solution is divided into two parts by a porous 

 partition, the cathode being on one side, the anode on the 

 other, both being immersed in the same solution, the substance 

 to be operated on being dissolved in the one compartment or 

 the other according to the effect, reduction or oxidation, which 

 it is intended to bring about. 



The Spectroscope is an instrument now familiar and to be 

 found in some form in every chemical laboratory. The dis- 

 covery that white light is made up of a great number of rays 

 which give to the eye the sense of colour, was made by Newton 

 at some time previous to the year 1675, when he described 

 many experiments, and gave the explanation of them in his 

 treatise on " Opticks " presented to the Royal Society. But 

 the application of this discovery to the purposes of the chemist 

 and physicist came nearly two hundred years later, when the 

 spectroscope was invented and used by Bunsen and Kirchhoff, 

 professors at the University of Heidelberg. Newton discovered 

 that lights which differ in colour, differ in refrangibility, and 

 when a beam of white light passes through a transparent prism 

 the coloured rays of which it is composed are spread out, and 

 when received on a white screen exhibit a coloured band called 

 the spectrum. In order that the coloured rays may not overlap 

 and confuse one another the light should be made to pass through 

 a narrow slit parallel with the edges of the prism. The spectro- 

 scope then is an instrument consisting of one or more prisms, 

 t'irough which the light is made to pass, and by which the coloured 

 rays are separated and dispersed. The light to be examined is 

 admitted through a slit and passes, on its way to the first face 

 of the prism, through a lens fixed in a tube, called the collimator, 

 by which the rays are rendered parallel. The spectrum produced 

 by passage through the prism is observed through a telescope 

 movable through a small arc so as to enable the observer to see 

 either the less refrangible rays at the red end or the more re- 

 frangible blue. The eye of the observer is sometimes replaced 



