EVOLUTION OF OASES FROM HOMOGENEOUS LIQUIDS. 295 



(iv.) Increase of pressure diminishes or even stops the evolution of a gas from a 

 liquid 



The chemical changes in the case of homogeneous liquids herein studied are the 

 decomposition of formic acid into carbonic oxide and water ; of oxalic acid into 

 carbonic oxide, carbonic acid, and water ; of potassium ferrocyanide, forming carbonic 

 oxide and water ; of nitric acid into nitric oxide ; of ammonium nitrate into nitrous 

 oxide and steam ; of ammonium nitrite into nitrogen and water ; of potassium 

 chlorate into the perchlorate and oxygen. 



The particular case of the formation of hydrogen from zinc and sulphuric acid 

 is also investigated. 



The finely divided chemically inert substances include pumice, silica, graphite, 

 barium sulphate, and finely divided glass. 



(v.) The particular case of the decomposition of formic acid into carbonic oxide and 

 water is also investigated ; the rate of change is shown to be directly proportional to 

 the mass of substance undergoing the change. The curve representing the interval 

 of time required for each unit of chemical change in terms of the mass present is 

 shown to be hyperbolic and illustrative of the law 



dr ,- 



This indicates that the change results between equivalent masses, probably of 

 formic acid itself, the sulphuric acid merely serving to induce the reaction between 

 two molecules of the acid, precisely as it serves to induce the etherification of an 

 organic acid by an alcohol. 



The observed values for the sum of the interval of time required for each unit of 

 decomposition taken as the standard are concordant with those calculated according 

 to the hypothesis adopted. So far as the investigation has been carried on no imme- 

 diate relation has been discovered between the rate of change and the temperature, 

 though the introduction of particles and the cleanliness, or otherwise, of the containing 

 vessel may have introduced errors not readily obviated. 



In presence of finely divided particles, as silica, the law governing this chemical 

 change is still valid, though the change itself occurs at a much lower temperature. 



In conclusion, I would again allude to my great obligations to Mr. VERNON 

 HARCOURT, and to Mr. W. ESSON, for help in the mathematical portion of Part III., 

 to whom I would express my best thanks. I would also acknowledge, with thanks, the 

 receipt of a grant from the Government Grant Committee of the Royal Society, to 

 assist me in this investigation. 



