216 MESSRS. A. V. HAECOUET AND W. ESSON ON THE LAWS OF CONNEXION 



established by them. Its truth, however, appears to be demonstrated in another and 

 simpler case of chemical change which we have since examined, and therefore it may 

 now be fairly applied to the elucidation of results which when first obtained were unin- 

 terpretable. 



On this simpler case — the reaction occurring between hydriodic acid and peroxide of 

 hydrogen — ^we have made numerous series of experiments, varying in succession each 

 condition of the reaction. The results of this investigation, which is now nearly com- 

 pleted, may, we hope, form the subject of a second communication. 



Appendix, 



containing a Theoretical Discussion of some cases of Chemical Change. 

 ^y William Esson, M.A., Fellow of Merton College, Oxford. 



The most simple case of chemical change occurs in a system in which a single sub- 

 stance is undergoing change in presence of a constant quantity of other substances, and 

 at a constant temperature. A practical constancy of the other substances is obtained 

 by having them present in large excess ; for any change produced in their amount by 

 reason of the change of the single substance is infinitesimal in comparison with their 

 original amount, and its effect on the system may therefore be neglected. 



By a " system " is meant a unit of volume in which given quantities of substances 

 are present; these quantities are called "elements of the system;" " a system in which 

 a single substance is undergoing change," is a system in which the variation of the 

 other substances does not affect the change of the single substance. 



It has been ascertained by experiment that the residue y of the substance under- 

 going change in a system of this kind, is connected with the time x during which the 

 change has been proceeding, by the following equation, 



y—ae-", (1) 



a being the quantity of the substance in the system at the commencement of the change, 

 and a a constant, the meaning of which may be thus determined; differentiating (1) and 

 eliminating x, we have 



l=-«^ (2) 



Now — ^ is the amount of substance which disappears in a unit of time at the time x, 



when^ is the quantity of substance present in the system, and the equation (2) expresses 

 the law that " the amount of change in a unit of time is directly proportional to the 

 quantity of substance ; " following the analogy of the motion of a material particle, we 



