184 CHKMICAL AFFINITY. 



when cold with so much avidity as to take fire and burn the 

 moment it is exposed to the air. Iron also, in the spongy and 

 divided state in which it is procured by reducing the peroxide 

 by means of hydrogen gas at a low red heat, absorbs oxygen 

 with equal avidity at the temperature of the air, and takes fire 

 and burns. But notwithstanding an affinity for oxygen of 

 such intensity, these metals in mass oxidate very slowly in 

 air, particularly lead, which is quickly tarnished indeed, 

 but the thin coating of oxide formed does not penetrate to a 

 sensible depth in the course of several years. The suspension 

 of the oxidation may be partly due to the comparatively small 

 surface which a compact body exposes to air, and which be- 

 comes covered by a coat of oxide and protected from farther 

 change ; but partly also to the effect of the conducting power 

 of a considerable mass of metal in preventing the elevation of 

 temperature consequent upon the oxidation of its surface. 

 For metals oxidate with increased facility at a high tempera- 

 ture, such as the lead pyrophorus quickly attains from the oxi- 

 dation of the great surface which it exposes, compared with its 

 weight. The heat from the oxidation of the superficial parti- 

 cles of the compact rnetal, however, is not accumulated, but 

 carried off and dissipated by the conducting power of the con- 

 tiguous particles, so that elevation of temperature is effectually 

 repressed. It thus appears that the state of aggregation of a 

 solid may oppose an insuperable bar to the action of a very 

 powerful affinity. 



The affinity of two bodies, one or both of which are in the 

 state of gas, is often promoted in an extraordinary manner by 

 the contact of certain solid bodies. Thus oxygen and hydrogen 

 gases may be mixed and retained for any length of time in that 

 state without exhibiting any affinity for each other, and the 

 gaseous mixture may, indeed, be heated in a glass vessel ta 

 any temperature short of redness without showing any disposi- 

 tion to combine. But if a clean plate of platinum be introduced 

 into the cold mixture, the gases in contact with the metallic sur- 

 face instantly unite and form water ; other portions of the mix- 

 ture come then in contact with the platinum and combine succes- 

 sively under its influence, so that a large quantity of the gaseous 

 mixture may be quickly united. The temperature of the 

 platinum also rises from the heat evolved by the combination 

 occurring at its surface, and the influence of the metal in- 



