294 PHYSIOLOGY AT THE FARM. 



fails. In oxygen gas the combustion goes on so rapidly as far 

 more than to compensate for what is lost by radiation and 

 conduction. Phosphorus and potassium, if previously heated, 

 burst into flame when put into oxygen gas. According to 

 recent views, the rapid motion between the atoms of the two 

 bodies, determined by their great affinity, when stopped by 

 their near approach, becomes changed into heat. Combustion, 

 then, is a mere accident of chemical combination between two 

 bodies ; that is, such combination is called combustion, when 

 the heat extricated is sufficient to give rise to light. If it be 

 asked why there are cases of chemical combination in which 

 cold is produced ? the answer is that such cases are uniformly 

 accompanied with liquefaction, as when snow and salt, or 

 snow and chloride of calcium, are mingled. There is undoubt- 

 edly in both examples an exercise of affinity, but in the transi- 

 tion from the solid to the fluid form of the solution there is 

 heat changed into motion whence the heat disappears to a 

 certain extent. 



When charcoal or carbon burns in oxygen gas, great heat 

 is produced, notwithstanding that the product, carbonic acid, 

 is gaseous. But the heat here produced would be greater if 

 the product were solid and of considerable density ; for the 

 actual heat, as measured, must be the heat extricated on the 

 interruption of the motion given to the atoms by the affinity, 

 diminished by the amount of heat converted into motion dur- 

 ing the expansion of the carbonic acid or the product. 



It is an important fact in physiology, that the heat produced 

 by the combination of a given quantity of oxygen with carbon, 

 to afford carbonic acid, is always the same under the most 

 different circumstances ; for example, the same whether the 

 combination takes place at a high temperature of the atmos- 

 phere, or slowly within the bodies of animals. 



To the slow combustion not merely of carbon but of other 



