92 CA TAL YSIS. PART i. 



it down. In such cases the potential energy of the 

 unstable mass is in a moment changed into vis viva or 

 impetus. Daguerreotype impression shows the power of 

 the chemical rays on substances in unsteady equilibrium, 

 and the length of time required to make the impression 

 under the same circumstances is a measure of the in- 

 stability. 



'Most of the fulminates are compounds of nitrogen ; of 

 that the fulminate of aniline is a recent instance, since 

 it is formed by the slow action of nitrous acid on ani- 

 line. Explosion takes place on the sudden evolution of 

 gas, or the sudden change of a solid into vapour. In 

 these cases fire or percussion are the foreign causes of 

 change. They are all particular instances of the 

 general principle of catalysis, which is the chemical 

 combination of heterogeneous atoms by the action of 

 a substance that does not participate in the change. 

 Thus it has long been known that when platinum is 

 plunged into a mixture of oxygen and hydrogen it 

 combines these gases into water. Acids in some cases 

 seem to have the same eifect ; for when rags or starch 

 are dissolved in an acid the starch is changed to dex- 

 trine and the liquid has acquired the power of turning 

 the plane of polarised light to the right. The acid has 

 undergone no alteration, but it has changed the proper- 

 ties of the starch though not its chemical composition. 

 After a time, a second transformation takes place, the 

 liquid ceases by degrees to turn the plane of polarisation 

 to the right, and ends by turning it to the left. The 

 acid is still unchanged, but the dextrine has now dis- 

 appeared : it has combined with the water and is 

 transformed into glucose or sugar of grapes. 



The quantity of the physical powers, active and 

 latent, is inappreciably great. The quantity of heat or 

 potential energy generated by chemical combination 

 alone is enormous. 



