COMBUSTION. 



321 



COMBUSTION. 



Many examples have been presented, in which the chemical combination 

 of two bodies was accompanied by a change of temperature. When sulphuric 

 acid and pure water are mixed together at the same temperature of 60°, the 

 mixture will suddenly rise to the temperature of boiling ^ater. In like man- 

 ner, when snow at the temperature of 32° is mixed w^n common salt at the 

 same temperature, the compound resulting will fall many degrees below the 

 common temperature of the constituents. It may b^ taken, therefore, as a ge. 

 eral principle, that chemical combination is or- of the numerous causes b) 

 which heat may be developed or absorbed, ivery part of chemical science 

 abounds in facts illustrative of this principle 



We have seen that an extreme increase of temperature is attended by the 

 presence of light. Now, if these two general laws be placed in juxtaposition, 

 it may be expected that, if chemical combinations can be discovered in which 

 extreme quantities of heat may be developed, the product may attain that tem- 

 perature at which it will be luminous. 



Such are the principles which form the foundation of the ordinary process 

 of combustion or burning. When fire is produced, such a combination always 

 takes place between the particles of two bodies as produces a development of 

 heat so extreme as to produce light. If the body emitting light in this case 

 have the solid form, the effect is called fire ; but if it be vapor, it is called 

 fiame. 



It so happens thac, among the infinite variety of natural substances by the 

 combination of whi-h this remarkable phenomenon is produced, one of the two 

 combining bodies is, almost in every case, the substance called in chemistry 

 oxygen gas ; and that in the few cases where oxygen is not present there is a 

 very limited number of other substances, one or the other of which must be one 

 of the combining substances. 



Among these other substances, the principal are three bodies, called in chem- 

 istry chlorine, bromine, and iodine. 



VOL,. II.— 21 



