RELATIONS TO PHYSICS AND CHEMISTRY 309 



direction being that of the evolution of heat. Yet this principle, 

 however weighty, is not absolutely 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 character 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 will be easy to repro- 

 duce 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 investi- 

 gations of Berthelot and Pean de St. Gilles on the mutual action of 

 acids and alcohols, and those of Deville and Debray on high tempera- 

 ture action, which even splits up water, have shown that many chem- 

 ical changes can be reversed. Indeed, we have types corresponding 

 absolutely to evaporation, as the loss of water-vapor from hydrates: 

 and others corresponding as well to freezing and melting, as the split- 

 ting 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 possi- 

 bility of equilibrium, the two chemically 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 influ- 

 ence of temperature, of electricity, of light, of pressure. And the eas- 

 iest 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 with the 

 water. We know that this chemical 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 bi- 

 sulphate. And it is possible in such cases exactly to determine the 

 limiting pressure, such that a higher one presses out the sulphuric 

 acid while a lower one is overpowered by the affinity action. If the 

 chemical change takes place under a pressure 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 this 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 affin- 

 ity made possible the application of a physical principle known as the 



