228 CHEMICAL DISCOVERY AND INVENTION 



compounds. These external electrons differ from the constitu- 

 tional electrons in the fact that their removal does not disturb 

 the essential structure of the atom as a whole. When they are 

 restored therefore there is no change of properties and the case 

 is different from that of the radio-active elements of which, 

 when they lose an electron, that is a /3 particle, the residue is 

 fundamentally changed in properties. 



On the assumption that these external electrons which deter- 

 mine valency move round the atoms to which they are attached 

 in small circles, Sir William Ramsay has very recently (Proceed- 

 ings of the Royal Society, July, 1916) been considering the 

 manner in which atoms, represented as spheres, would place 

 themselves in the formation of some of the simpler molecules, 

 hydrogen, oxygen, hydrogen chloride, water, ammonia. So far 

 as they go the results are consistent with previous ideas, but the 

 difficulties increase seriously when any attempt is made to 

 represent any but the very simplest cases of molecular configura- 

 tion. 



CHAPTER XIII 



COLLOIDS 

 THE MICROSCOPIC AND ULTRAMICROSCOPIC 



HUMAN existence hangs between two great worlds, the infinitely 

 great and the infinitely little, and into both the chemist can 

 penetrate. Though the mind fails altogether to grasp any clear 

 idea of the space which separates us from any of the stars we 

 can at least realise that it is enormous by a very simple reflec- 

 tion. As the earth travels round her orbit she sweeps out a 

 circle more than 180 millions of miles in diameter. Nevertheless 

 an observer who looks out into the night in summer and again 

 in winter, or in spring and again in autumn, will be able to dis- 

 tinguish no change in the relative positions of the stars forming 

 the constellations or patterns which they have for so many ages 

 written on the background of the sky. And when we remember 

 that though modern instruments are capable of appreciating 

 differences far more minute than any which the unaided eye 

 can trace, the vast majority of stars remain apparently un- 

 altered in position when viewed from opposite sides of the earth's 

 orbit, Into these vast spaces the spectroscope, brought into use 



