126 PROGRESS OF SCIENCE IN THE CENTURY. 



When two different substances are brought to- 

 gether it frequently happens that changes occur re- 

 sulting in the production of a new substance or sub- 

 stances. Thus an acid and an alkali, as noted above, 

 produce a salt. Since the indestructibility of matter 

 was recognised, and since Dalton made the atomic 

 conception current coin, it has been evident that the 

 change occurs through a separation and re-combina- 

 tion of the component particles of the two substances. 

 As Dalton said : '^ All the changes we can produce 

 consist in separating particles that are in a state of 

 cohesion or combination, and joining those that were 

 previously at a distance. '^ But after the phenomena 

 of change have been observed, the question is bound 

 to arise — why should the atoms separate and re-com- 

 bine at all ? Is the phenomenon comparable to any- 

 thing else in our experience, or is ' chemical affinity ' 

 an irreducible fact ? Masses attract one another and 

 we can measure the force; is chemical affinity also 

 measurable and does it bear any analogy to gravita- 

 tion ? There is also attraction due to magnetism 

 and different electrical states; has chemical affinity 

 anything to do with this ? Thus arises the inevitable 

 problem of chemical affinity ; it is still unsolved, but 

 we may profitably consider for a little some of the 

 suggestions which have been offered. 



It is part of the work of chemistry to distinguish 

 the different kinds of matter, and we began this his- 

 torical sketch by alluding to the search for the ele- 

 ments; but a more important problem is to interpret 

 chemical affinity, or the capacity of the elements to 

 exert chemical action. 



Electricity and Chemical Affinity. — In the long 

 history of attempts to interpret the chemical activi- 

 ties of different kinds of matter in their relations to 



