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 change! w 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 f 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 



