22 COMBUSTION. 



the supporter has for caloric, which keeps it in a gaseous 

 state ; it therefore unites with the ignited body, and 

 allows the caloric to escape. The rapidity of combustion 

 is also found to be in proportion to the quantity of its 

 supporter; this is evident from a common culinary fire, 

 which increases in intensity in proportion to the supply 

 of air it receives. 



In combustion in common air the oxygen is always 

 consumed ; and the weight of the products of every 

 combustion is equal to the weight of the body before 

 combustion, together with that of the oxygen consumed; 

 so that instead of a body being destroyed or burnt away, 

 as it was formerly termed, it is found to increase in 

 weight, and that in some instances to an almost incon- 

 ceivable amount. One ounce of phosphorus, for instance, 

 if properly burnt, will produce very nearly two ounces 

 of phosphoric acid ; and 10D Ibs. of metallic lead, when 

 made into red lead, which it is by being caused to im- 

 bibe oxygen through the medium of a reverberatory fur- 

 nace, will weigh 1 12 Ibs. 



It used to be thought that the light evolved during 

 combustion came from the oxygen ; but it has been 

 latterly supposed to come from the combustible body, 

 as the colour of the light depends in all cases on the 

 nature of the body. The combustible body is, accord- 

 ing to this theory, said to afford light and the base, 

 oxygen gas, oxygen and caloric ; the oxygen will then, in 

 combustion, unite with the base, and generally form an 

 oxide, and the caloric of the oxygen will unite with the 

 light and form fire and light. Although the product of 

 combustion is generally an oxide, it is not always so ; 

 sometimes it is an acid, as phosphorus and sulphur, when 

 burnt in oxygen gas or atmospheric air, produce phospho- 

 ric and sulphurous acid. The product of potassium* 



* Potassium is considered a metal, and is the lightest known, being 

 lighter than water. It does not exist naturally in a metallic state, but 

 is formed by voltaic action on an hydrate of potash. It is of a per- 

 fectly white colour, and has the brilliancy of silver, but through its 

 affinity for oxygen, which is greater than that of any other known 

 body, it cannot be exposed an instant to the atmosphere without be- 

 ing oxidized. If a small piece be laid upon water, it will attract the 



