166 Mr. V. H. Yeley on the Phases arid 



problems of the commencement of chemical change, and espe- 

 cially with regard to the function and necessity of an inter- 

 mediate or third substance to promote reaction between two 

 other substances. 



But generally there seems to be a tendency among writers 

 on chemical as on other forms of philosophy to accept the 

 hypotheses of their predecessors as matters of acquired know- 

 ledge without due examination of their validity, and to super- 

 impose upon them other hypotheses which their successors in 

 their turn accept as facts. In chemical philosophy, as in that 

 of pure reason, there are writers of a dogmatic type who set 

 forth w r ith a most unqualified assurance hypotheses neither 

 wholly in accordance with facts nor strictly logical in their 

 demonstration. But others of an agnostic type are content 

 to conceal their ignorance under such commonly accepted 

 terms as "catalysis," "fermentation/' "nascent action," and 

 the like, which are regarded as all sufficient explanations of 

 phenomena not understood ; such terms are of course con- 

 venient, but do not render any material assistance. Again, 

 it is to be feared that chemists too often content themselves 

 with a study of the ultimate products of a reaction and omit 

 the intermediately formed substances, which, though in some 

 cases they may be transitory, yet are the more important. It is 

 as if the morphologist were to consider an organism as a whole, 

 and to ignore the structure and function of individual cells, 

 whether somatic or reproductive. In the present communi- 

 cation it is proposed to discuss the conditions and phases of 

 chemical change, and to deal with the several hypotheses 

 concerning its nature. 



The Phases of Chemical Change. 



Prom our earliest and everyday experience it is learnt 

 that chemical changes start slowly at first, are accelerated up 

 to a maximum and for some time constant stage, and thence, 

 unless reversible, proceed with a velocity diminishing accord- 

 ing to the proportion of the reacting masses. To adopt the 

 phraseology of the biologist, the changes are at first anabolic 

 and then katabolic. There are thus a series of phases or 

 periods which pass into one another more or less continuously ; 

 and though in some cases, especially of detonations and cer- 

 tain explosions ; all these periods have not as yet been clearly 

 defined owing to the rapidity of their succession, it is, how- 

 ever, reasonable to suppose that with improved methods of 

 observation, whether of chronography or photography, these 

 changes will be found to follow the same course. 



