146 COMBUSTION. 



of which are by the access of atmospheric air, by the de- 

 composition of water, and by the decomposition of acids. 

 Iron may be mentioned as a familiar example of a metal be- 

 coming oxydized by atmospheric air. It is well known that 

 when this metal is exposed to air and moisture, it acquires 

 rust, or in other words its surface is converted to an oxyd, 

 in which state the metal will be found to have acquired an 

 increase of weight. Common red lead, which is a true oxyd 

 of lead, is made by melting that metal in ovens so constructed 

 as to have a free access to atmospheric air. Gold, silver, 

 and platina, cannot be oxydized, unless in a very high tem- 

 perature ; and with respect to other metals, they not only 

 differ in their capacity for oxygen, but also in their attrac- 

 tion for it ; so that one will often rob the other, thus reduc- 

 ing the first oxyd to its primitive metallic form. If you 

 dissolve some quicksilver in nitric acid, and after dropping 

 a little of the solution upon a bright piece of copper, gently 

 rub it with a piece of cloth, the mercury will precipitate it- 

 self upon the copper, which will be completely silvered. 



With regard to the oxyds of nitrogen ; the first degree 

 of oxydizement produces nitrous oxyd ; — a further portion 

 of oxygen nitric oxyd, and they are both in a state of gas. 

 Nitrous oxyd gas bears the nearest resemblance of any other 

 to atmospheric air. It will support combustion even better 

 than common air ; it is respirable for a short time, and it is 

 absorbed by water. Persons who have inhaled this gas have 

 felt sensations similar to those produced by intoxication. In 

 some people it produces involuntary muscular motion and a 

 propensity to leaping and running ; in others, involuntary fits 

 of laughter ; and in all, high spirits, and the most exquisite- 

 ly pleasurable sensations, without any subsequent feelings of 

 debility. It is readily procured by exposing crystals of ni- 

 trate of ammonia, in a retort, to the heat of a lamp, by 

 which means, the ammoniacal salt is decomposed, and this 

 gas is evolved. 



Combustion may be defined to be a process by which cer- 

 tain substances decompose oxygen gas, absorb its base, and 

 sutler its caloric to escape in the state of sensible heat. The 

 agency of oxygen in combustion is attributable to its affinity 

 for combustible bodies. The combustible having a greater 

 affinity to oxygen than oxygen has to caloric, the oxygen 

 gas is decomposed, and its oxygen combines with the ignited 



