October 2, 1896.] 



SCIENCE. 



469 



an opportunity is given for the escape of 

 the negative charge from the zinc, and 

 at the same time positive ions are al- 

 lowed to escape from the solution at 

 some other point, the zinc will continue 

 to dissolve and currents of electricity will 

 be set up. Thus, in the Daniell, or gravity 

 cell, zinc ions pass into solution and a 

 corresponding number of copper ions are 

 deposited. The force which causes the 

 movement of the ions, and with them the 

 transference of electrical energy within the 

 cell, is mainly the very high solution pres- 

 sure of the zinc as compared with that of 

 copper. Other factors, such as the osmotic 

 pressure of zinc ions already in solution, 

 which tends to conteract the solution pres- 

 sure of the zinc, the osmotic pressure of 

 copper ions which aids in the separation 

 of the copper, and the different velocity of 

 translation for various ions which may 

 cause differences of potential when the 

 fluids of the cell are not homogeneous, are 

 most of them comparatively small in their 

 effect. 



No means has been found for the direct 

 determination of the solution pressure of 

 metals, but it may be calculated from the 

 difiference in potential between a metal and 

 a solution of one of its salts. Methods for 

 the determination of the latter have been 

 devised by Ostwald,^^ and improved by 

 Paschen.^® By the use of these and other 

 constants which the researches of physical 

 chemistry have placed in his hands, the 

 physicist can now calculate the electromotive 

 force which can be obtained by various 

 combinations of metals and solutions. On 

 this side the theory has rendered essentially 

 the same service for the galvanic cell which 

 the atomic theory rendered for chemical 

 compounds when it furnished the means for 

 calculating their percentage composition. 

 As in the early days of the atomic theory, 

 many of the constants in question are im- 

 perfectly known, but since the theory has 



shown clearly their interdependence, new 

 means for their determination and for the 

 control of their accuracy are constantly be- 

 ing discovered. 



Every one who is familiar with the ex- 

 tremely wasteful character of all processes 

 now at our disposal for the transforma- 

 tion of chemical into mechanical energy 

 must have had the thought that there is 

 surely some means of saving a part of the 

 enormous loss. At present the attention 

 of the scientific world is turned toward the 

 transformation of the chemical energy of 

 coal into electrical energy as the probable 

 solution of this problem. It seems to be 

 almost certain that physical chemistry has 

 already made clear the principles by means 

 of which such a transformation may be ac- 

 complished. Indeed, Dr. W. Borchers," by 

 the use of a solution of cuprous chloride 

 with producer gas, or carbon monoxide on 

 one side and air on the other, has already 

 obtained an electrical current which corre- 

 sponds to a transformation of thirty per 

 cent, of the chemical energy into electrical. 

 This is an efficiency three times that of the 

 best steam engines. There is no probabil- 

 ity that this method can ever be practically 

 useful, but that a practical method will 

 soon be discovered is, at least, possible. 



I have thus far spoken of the achieve- 

 ments of physical chemistry mainly in the 

 direction of the development of the theories 

 of osmotic pressure and of electrolytic dis- 

 sociation. It is in this field that the most 

 valuable practical results have been secured, 

 because it is here that a new, far-reaching, 

 and extremely useful theory has been de- 

 veloped. But work in physical chemistry 

 has been extremely active in many other 

 directions as well. 



The most brilliant chemical discovery of 

 the last decade was a result of the careful 

 study of a single physical property of nitro- 

 gen. And, owing to the peculiar character 

 of argon and helium, their further study 



