EVOLUTION OP OASES FROM HOMOGENEOUS LIQUIDS. 285 



The rate of evolution of gas is at first very slow, and then quickly increases ; 

 on reducing the pressure (Series II.) there is a considerable increase in the rate; on 

 allowing the gas to fill up the vacuum the rate is slightly decreased, and on again 

 reducing the pressure the rate is again slightly increased. 



Another series of experiments gave precisely similar results, which seems to indicate 

 that in this case the rate of evolution of gas is influenced to a slight degree by 

 reduction of pressure, a result which differs from those observed in previous cases of 

 the formation of a gas in a liquid But this result may be due either to an accelera- 

 tion of the chemical change taken in itself, or to the greater agitation of the liquid 

 by the formation of larger bubbles of gas, whereby the liquid already acted upon in 

 the immediate vicinity of the metal is more readily removed, and thus the metal comes 

 more intimately in contact with fresh layers of the acid. 



From these results it is apparent that reduction of pressure from one to a fraction 

 of an atmosphere does not permanently alter the rate of evolution of a gas from a 

 liquid, but that an increase of pressure produces temporarily a retardation, or even 

 complete stoppage, of the evolution of gas, and, conversely, a decrease of pressure 

 produces an acceleration. The only apparent explanation of these phenomena seems 

 to lie in a storage, to some small extent, of the gas within the liquid, which is 

 increased or decreased temporarily by increase or decrease of pressure ; when matters 

 are so equalised that the possible storage of gas has reached its maximum, then the 

 evolution of gas would be the same, whatever be the pressure to which the liquid is 

 subjected. The effect produced by increasing the pressure is the same as that of 

 lowering the temperature. 



It would seem, however, that such a gas as carbonic oxide at the moment of its 

 liberation in the so-called nascent state is of a kind different from the same gas ready- 

 formed, in that the one is susceptible of solution in sulphuric acid, while the other is 

 practically insoluble. 



PART m. 

 THE RATE OF DECOMPOSITION OF FORMIC ACID INTO CARBONIC OXIDE AND WATER. 



In the introductory sketch, remarks were made on the unsuitability of many 

 chemical changes leading to the formation of a gas for an investigation on the 

 variations in the velocity of such a change produced by alterations in the masses of 

 reacting substances, as also by changes in external conditions, such as temperature, 

 and material of containing vessel. It is difficult also to find cases in which are 

 fulfilled the conditions necessary for such an investigation, namely, that the masses of 

 all but one of the reacting substances should remain constant within small and 

 inappreciable limits. Another difficulty presents itself in the estimation of the 

 residue of unaltered material at the conclusion of any one of the observations, without 



