308 PHYSICAL CHEMISTRY 



charge, and so the valency of any element might be reduced to unity. 

 The latest supposition, that matter is built up of electricity alone, lies 

 again beyond the scope of this address. 



Let me now turn to the second part of my subject, and touch upon 

 the problem of affinity; indeed, the action that keeps atoms together 

 must be closely related to affinity. 



II. Physical Chemistry and our Notions concerning Affinity 



While physical chemistry, in the first period of its development, 

 was chiefly devoted to the study of the physical properties of matter, 

 the second and present period is characterized by the predominant 

 place of the problem of affinity. 



This change in the general aspect of our science goes hand in hand 

 with a different way of working : in the development of our ideas of 

 matter, physical chemistry introduced physical methods and instru- 

 ments for the study of physical properties; in the development of our 

 ideas of affinity, physical chemistry has introduced physical prin- 

 ciples. 



Affinity considered as Force. The first line of thought considered 

 affinity as a force, and in this direction it was natural to think of the 

 Newtonian attraction as the chemical agent. So it was that Berthollet , 

 and with far more success Guldberg and Waage, applied the laws of 

 mass-action to problems of affinity, formulating a relation still known 

 as the mass-law, according to which affinity is proportional to the 

 weight in the unit of volume. 



Now, as we all know, affinity is of a specific nature, and does not 

 depend on weight merely; on the contrary, the least heavy elements 

 are generally the most active. So Berzelius built up his system 

 founded on the notion that elements have a specific electrical charac- 

 ter, either positive or negative, and, in combining, act by electrical 

 attraction. In this direction Helmholtz made a further step in taking 

 into account the quantitative side. Considering the electrical charges 

 involved in Faraday's law, he pointed out as very important that the 

 attraction due, for instance, to the negative charge in chlorine and 

 the positive one in hydrogen far exceeds the gravitational attraction 

 of the masses. Yet a satisfying notion of affinity was not obtained in 

 this way. 



Affinity measured as Work. A second line of thought took into 

 consideration not the force but the work that affinity represents; and 

 it seemed a decisive step when Thomson and Berthelot declared that 

 the heat developed in chemical change corresponds to the work that 

 affinity can produce. Indeed, it was in this way that in many cases an 

 a priori calculation of the heat development of a reaction permitted 

 prediction of the direction in which the process would proceed, the 



