286 MR. V. H. VELEY ON THE CONDITIONS OF THE 



producing complications in the chemical change itself. Further, the phenomenon of 

 initial acceleration, discussed fully in Part II., and so marked in all the chemical 

 changes investigated, introduces complications. The gas formed must not only be 

 practically insoluble in the liquid from which it is produced, but also in the liquid over 

 which it is collected. The only gases which fulfil the latter condition for the convenient 

 liquid, water, are nitrogen, nitric oxide, hydrogen, and carbonic oxide. Chemical 

 changes leading to the production of the last-named were more particularly inves- 

 tigated. The first trials were made on the rate of formation of carbonic oxide from 

 potassium ferrocyanide when dissolved in concentrated sulphuric acid; but these 

 experiments were soon discarded, for this reaction was found to be complicated initially 

 by the formation of hydrocyanic acid, and, subsequently, by that of sulphurous oxide. 

 The temperature at which this reaction takes place is also inconveniently high. 



The decomposition of formic acid into carbonic oxide and water by means of 

 sulphuric acid was then selected as a suitable case for investigation. In preliminary 

 experiments it was ascertained that either from the acid or from its sodium salt the 

 volume of carbonic oxide required theoretically for the equation 



HCOOH = CO + H 2 



was given off at conveniently low temperatures without any separation of carbonaceous 

 matter, and consequent reduction of the sulphuric acid. In fact the determination of 

 volume of carbonic oxide evolved affords a ready method for the estimation of the 

 quantity of formic acid or of a formate, and since these experiments this method has 

 been applied by Messrs. HABCOUBT and POULTON to determine the amount of this 

 acid in the secretion from Lepidopterous larvae.* 



The twin U-tube arrangement was used, and, as explained above, 10 c.c. of dry air 

 at C. and 760 mm. pressure was taken as the standard. Thus, each time that the 

 chemical operation represented by the equation 



HCOOH = CO + H 2 O 



was repeated '02063 gram formic acid disappeared, with production of 10 c.c. or 

 '01256 gram carbonic oxide and '00807 gram of water. 



Obviously, for exact observations, it is required to fulfil two rather incompatible 

 conditions, (i.) that the sulphuric acid shall be sufficiently concentrated to effect the 

 decomposition, and (ii.) that it shall be sufficiently dilute for the amount of water 

 formed during the whole course of the operation not materially to dilute the acid. 

 Thus, the mass of dilute sulphuric acid must be relatively large, and that of the formic 

 acid relatively small. 



* 'Brit. Assoc. Report,' 1887, p. 766. 



