PAPERS OX CHEMISTRY AND PHYSICS 159 



determined. Now oxidation is based on the ability to 

 lose electrons so that whether it will take place in a given 

 case can be predetermined by referring to the electro- 

 motive series which lists metals in the order of their 

 ability to lose electrons. 



Heliimi is an element which f orms no compounds. The 

 chemist would say that this is because it is inactive, but 

 the physical chemist has gone much farther when he ex- 

 plains it as due to the inability of helium to lose electrons 

 and thus gain a positive valence, while the phenomenon 

 is entirely accounted for by the sub-atomic physicist who 

 says that the helium atom is composed of a positive neu- 

 cleus and negative electrons which swing in an orbit 

 which is in perfect equilibrium, so that there is no ten- 

 dency for an electron to leave it. 



Probably the most obvious contribution of physics to 

 chemistry is in apparatus embodying physical principles. 

 Whenever chemical changes or properties are not direct- 

 ly observable by the senses, the physical instrument is a 

 necessary medium. When the hands cannot detect heat 

 we use a thermometer; when the eyes cannot perceive the 

 arrangement of the atoms within a crystal, we use X-ray 

 diffractions to interpret this arrangement. 



The spectroscope is a true product of the inter-relation 

 of physics and chemistry, first because it was the out- 

 come of joint research by a physicist, Kirchoff, and a, 

 chemist, Bunsen, working together in the same labor- 

 atory, and second because it has found application in 

 both fields. To illustrate, Bunsen applied it to the analy- 

 sis of water of certain springs and thereby discovered 

 two new elements, caesium and rubidium. In the hands 

 of Kirchoff it explained the dark lines in the sun 's spec- 

 trum as being due to absorption, and as a result it became 

 the means of determining the composition of other plan- 

 ets. 



Another physical instrument of value to the chemist is 

 the polarimeter. This apparatus is used for measuring 

 the degree which an optically active substance will rotate 

 the plane of polarized light. Its importance to chemis- 

 try came when it was found that some substances with 



