234 



SCIENCE. 



[Vol. IV., No. 84. 



particles, — in other words, any change which results 

 in a modification of attracting force, — whether gravi- 

 tative or the commonly called chemical attracting 

 forces, results in an electrical potential; and con- 

 versely, that the passage of electricity through any 

 medium produces a change of aggregation of the mole- 

 cules and atoms. If we suppose that radiant energy 

 is electro-magnetic, cannot we suppose that it is ab- 

 sorbed more readily by some bodies than by others, 

 or, in other words, that its energy is transferred, so 

 that with the proper sense we would perceive what 

 might be called electrical color, or, in other words, 

 have an evidence of transformations of radiant energy 

 other than that which appeals to us as light and 

 color? We have arrived at the point in our study of 

 electricity where our instruments are too coarse to 

 enable us to extend our investigations. Is not the 

 physicist of the future to have instruments delicate 

 enough to measure the heat equivalent of the red and 

 the yellow and the blue violet rays of energy? in- 

 struments delicate enough to discover beats of light as 

 we now discover those of sound? The photographer 

 of to-day speaks in common language of handicap- 

 ping molecules by mixing gums with his bromide of 

 silver, in order that their rate of vibration may be 

 affected by the long waves of energy. Shall we not 

 have the means of obtaining the mechanical equiva- 

 lent of such handicapped vibrations? We have ad- 

 vanced; but we have not answered the question 

 which filled the mind of Franklin, and which fills 

 men's minds to-day: What is electricity? 



CHEMICAL AFFINITY. 1 



Professor Langley first reviewed the history of 

 chemical theory, and called attention to the final ex- 

 tinction of the term 'affinity' in the chemical litera- 

 ture of the present day. 



Shortly after the opening years of the present cen- 

 tury, three general methods were indicated for the 

 study of the force of affinity. Instead of being suc- 

 cessively taken up and abandoned, like all preceding 

 speculations, they have remained steadily in use dur- 

 ing the eighty years which have intervened, and to-day 

 they are still the most promising means at our dis- 

 posal. These three methods may be called the ther- 

 mal, the electrical, and the method of time or speed. 

 It will be convenient to consider each one separately. 



The most important generalization to be drawn 

 from thermo-chemical phenomena is, that the work of 

 chemical combination, or the total energy involved 

 in any reaction, is very largely influenced by the sur- 

 rounding conditions of temperature, pressure, and vol- 

 ume ; and the conclusion they force upon us in regard 

 to the nature of affinity is most important, namely, 

 that this force in accomplishing work is dependent, 

 like all other forces, on the conditions exterior to the 

 reacting system which limit the possible amount of 



1 Abstract of an address to the section of chemistry of the 

 American association for the advancement of science, at Phila- 

 delphia, Sept. 4, by Prof. J. W. Langlet, of the University of 

 Michigan, Ann Arbor, Mich., vice-president of the section. 



change. Affinity is therefore at last definitely re- 

 moved from the category of those mystical agents, so 

 often imagined by our predecessors in a less critical 

 age, which had no correlation with the general forces 

 of nature. 



Under the title ' dissociation,' St. Claire Deville 

 gave to the chemical world, in 1857, a new and fruit- 

 ful method of investigating the nature of compounds 

 by determining the temperature at which bodies 

 break up or are dissociated. The laws developed by 

 Deville and his successors in this field show us, that, 

 after the point is reached at which decomposition 

 commences, the further breaking up is determined 

 by the pressure of the evolved products of the re- 

 action, so that the permanence of the body depends 

 on the magnitude of two variables, pressure and tem- 

 perature, either of which may be varied at will 

 through a wide range. 



The electrical method of dissecting chemical forces 

 has been followed less actively than the thermal one. 

 Besides the well-known experimental contributions 

 of Davy, Becquerel, and Faraday, may be mentioned 

 Joule's researches on the heat absorbed during elec- 

 trolysis, and especially the work of C. K. Adler Wright, 

 on the 'determination of affinity, as electromotive 

 force.' The general outcome of these researches is, 

 that the products of electrolysis are so numerous, and 

 so varied by the results of secondary actions, that it is 

 very doubtful whether the electromotive force meas- 

 ured is that due solely to the union of those atoms 

 which are indicated by the principal equation of the 

 reaction. 



The method of time or speed of chemical reactions 

 has a history as old as that of its two associates ; but 

 the story is much less eventful, for very little work 

 has been done in this field. The most notable work 

 has been clone by Gladstone and Tribe, by ascer- 

 taining the rate at which a metallic plate could pre- 

 cipitate another metal from a solution. 



To these general methods for studying the problems 

 of chemical dynamics, should be added the investiga- 

 tion of the action of mass, by Gladstone, in his well- 

 known color work on tiie sulphocyanide of iron; of 

 the chemical action of light, by the late J. W. Draper 

 in this country, and Prof. H. E. Roscoe in England, 

 as well as Becquerel in France, — pioneers who have 

 since been followed by a host of students of scientific 

 photography. 



In the review just given, no attempt has been 

 made to do more than glance at the important con- 

 tributions to the theory and methods of measuring 

 affinity. Many names have been passed by, and 

 much work has been necessarily ignored. 



The history of the various modifications and ad- 

 ditions which have been made to the primitive 

 conception of the nature of affinity, when briefly 

 summarized, appears to be this: Hippocrates held 

 that union is caused by a kinship, either secret or 

 apparent, between different substances. Boerhaave 

 believed affinity to be a force which unites unlike 

 substances. Bergman and Geoff roy taught that union 

 is caused by a selective attraction ; and therefore they 

 called it ' elective affinity.' Wenzel and his success- 



