chemical change by measurement of the gases evolved. 581 
presence of these varied bodies, whereas the true explanation is 
that the gas stored up as a supersaturated solution is rapidly 
given off from the surface of the particles introduced. Veley 
also draws special attention to “ periods of induction ” in reactions 
A 
Fig. 1. 
investigated by the rate of evolution of gases, and suggests that 
these may be explained by supposing that there are intermediate 
products in the reaction whose formation occupies time at the 
beginning of the reaction. But a more general cause is necessary 
to explain a general phenomenon. The explanation is however 
38—2 
