November 24, 1905.] 



SCIENCE. 



653 



heat developed in chemical change corre- 

 sponds to the work that affinity can pro- 

 duce. 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 direction being 

 that of the evolution of heat. Yet, this 

 principle, however weighty, is , not abso- 

 lutely reliable. The chemical actions that 

 produce cold, as that of hydrochloric acid 

 on sodium sulphate, are objections not to be 

 overcome. 



The step really leading to a clear and 

 unobjectionable notion of affinity was made 

 in the study of the so-called reversible 

 chemical changes. This reversible char- 

 acter perhaps needs some explanation, 

 easily to be provided by an illustration. 

 Kill a chicken and prepare chicken soup ; 

 it would then be very difficult to get your 

 chicken again. This is because preparing 

 chicken soup is not reversible. On the 

 contrary, let water evaporate or freeze; it 

 Mall be easy to reproduce the water. 



Now, at first sight, chemical change does 

 not seem reversible; and indeed it often is 

 not, as in the explosion of gunpowder. 

 But investigations of Berthelot and Pean 

 de St. Gilles on the mutual action of acids 

 and alcohols, and those of Deville and De- 

 bray on high temperature action, which 

 even splits up water, have shown that many 

 chemical changes can be reversed. In- 

 deed, we have types corresponding abso- 

 lutely to evaporation, as the loss of water 

 vapor from hydrates ; and others corre- 

 sponding as well to freezing and melting, 

 as the splitting of double salts into their 

 components at definite temperatures, e. g., 

 copper calcium acetate at 77° C. Also in 

 analogy with physical phenomena, we have 

 in these reversible chemical changes the 

 possibility of equilibrium, the two chem- 

 ically different forms of matter coexisting, 



as do water and its vapor at a maximum 

 pressure. 



Such a reversal of chemical change can 

 take place under the influence of tempera- 

 ture, of electricity, of light, of pressure. 

 And the easiest way to arrive at a measure 

 of affinity is presented in the last case, as 

 was foreseen by Mitscherlich. Let us take 

 gypsum as an example. Burnt commercial 

 gypsum, mixed with water, will combine 

 wath the water. We know that thisi chem- 

 ical change can produce pressure, and that 

 it may be prevented by sufficient pressure 

 and be reversed by it, as Spring succeeded 

 in pressing out sulphuric acid from sodium 

 bisulphate. And it is possible in such 

 cases exactly to determine the limiting pres- 

 sure, such that a higher one presses out the 

 sulphuric acid while a lower one is over- 

 powered by the affinity action. If the 

 chemical change takes place under a pres- 

 sure only slightly less than that which 

 would prevent it, thus practically taking 

 place under the limiting pressure, we get 

 out of affinity the greatest quantity of work 

 that it can possibly produce ; and thils 

 quantity is the same whatever the nature 

 of the opposing action, be it electricity, 

 light, or anything else. Therefore, in this 

 maximum work we have a sound measure 

 of affinity. 



It was a very happy coincidence indeed,, 

 that this conception of affinity made pos- 

 sible the application of a physical principle 

 known as the second law of thermo- 

 dynamics. This principle may be formu- 

 lated in different ways. For my purpose 

 let me say that it limits the possibility of 

 natural processes to the occurrence of those 

 in which a difference of intensity is dimin- 

 ished. If there is a difference of pressure 

 in two parts of a gas, a movement will 

 occur producing equality ; if there is a dif- 

 ference of temperature, heat will be trans- 

 ported so as to produce equality once 

 more. It is curious that such simple neees- 



