528 THE POPULAR SCIENCE MONTHLY. 



between unlike atoms is a superior sort of cohesion, powerful and ab- 

 solute ; and this force was thought to operate between two elementary- 

 bodies directly, without the intervention of a third kind of matter. 

 That this so-called affinity is radically affected by physical state, by 

 heat, and by electricity, has been admitted, but the conviction is grow- 

 ing in the minds of chemists that many circumstances influencing the 

 union and separation of elements have been overlooked ; they are be- 

 ginning to believe that chemical action does not take place between 

 two substances, and that the presence of a third body is important, if 

 not, indeed, indispensable. Many years ago the word catalytic was 

 coined to describe certain isolated phenomena little understood. These 

 phenomena are familiar to chemists, and the number is increasing; the 

 word catalytic is, however, in disfavor, and the term contact-actions is 

 now current. The well-known influence of finely divided and heated 

 platinum in effecting the union of sulphur dioxide and oxygen and 

 the action of metallic silver in decomposing ozone without itself un- 

 dergoing any change are examples. In these and similar changes one 

 of the substances indispensable to the reaction remains unchanged, and 

 its role can not be expressed in equations. 



There is another class of reactions in which one body acts upon 

 another only through the aid of a third, which maintains its identity 

 at the close of the reaction, yet is known to be decomposed and re- 

 composed successively throughout the operation. By heating a rela- 

 tively small quantity of cobaltous oxide with bleaching-powder, the 

 latter is wholly decomposed, yielding calcium chloride, water, and 

 oxygen, yet at the close of the reaction the cobaltous oxide is found 

 unaltered. It has been shown that it is successively decomposed and 

 recomposed during the operation. In their investigation on " Simul- 

 taneous Oxidation and Reduction by means of Hydrocyanic Acid," 

 Professors Michael and Palmer consider it probable that many of the 

 most important reactions of animal and vegetable life are due to the 

 intercession of substances which undergo change during the reactions, 

 and in the end return to their original form. They suggest also that 

 some of these reactions seem to be dependent on substances capable of 

 decomposing water into its elements, or into hydrogen and hydroxyl ; 

 and, when the chemist can command a reagent possessing that prop- 

 erty at a low temperature, their imitation in the laboratory may follow 

 its discovery. 



That chemically pure zinc is not soluble in dilute sulphuric acid 

 has been known since Faraday's day ; that sodium does not combine 

 with perfectly dry chlorine, even if the metal be heated to its fusing- 

 point, was shown by Wanklyn in 1869 ; more recently, Mr. Cowper 

 has found that dry chlorine does not attack Dutch metal ; six years 

 ago, Mr. H. B. Dixon demonstrated before the British Association 

 that a well-dried mixture of carbon monoxide and oxygen can be sub- 

 jected to the electric spark without exploding. In March, 1885, Mr. 



