158 ATTRACTION. 



in the nascent state, will unite with another already in the elastic 

 form ; thus hydrogen unites with nitrogen, to form ammonia, and 

 both these gases, evolved from putrefaction, combine in their nascent 

 state, and form the same body. 



Mechanical force favors the combination of gases with each other, 

 and with fluids ; oxygen and hydrogen can be made to combine by sud- 

 den and violent pressure ;* and pressure, cold and agitation are the usual 

 means of impregnating fluids with gases, as in the case of soda water. 



Elasticity favors decomposition. When one constituent of a body 

 is prone to assume the aerial state, in general that body is more easily 

 decomposed, both by heat and by affinity, than if both ingredients were 

 fixed. This is the case with the carbonates, and with water contain- 

 ed in crystals, and other combinations ; and even potassium is driven off 

 by its superior volatility, at an intense heat, when the alkali contain- 

 ing it is brought into contact with highly ignited iron. Many instan- 

 ces in illustration of these views will occur as we proceed. 



6. Efflorescence. This is a circumstance of no great importance, 

 but it sometimes favors- chemical action, by withdrawing a salt that has 

 been formed, from the field of action, and in this manner leaving the 

 remaining ingredients free to act again. Thus in the country around 

 the natron lakes in Egypt, muriate of soda and carbonate of lime mu- 

 tually decompose each other, and the carbonate of soda crawls up in 

 crystals upon the grass and other bodies accidentally present. A 

 similar effect I have often observed upon common plaster, made 

 with sea sand containing muriate of soda, which undergoes decompo- 

 sition, with the carbonate of lime, and forms by efflorescence a plu- 

 verulent carbonate of soda appearing like a fine snow upon the walls. 



7. Temperature. The relation of bodies to heat is of the utmost 

 importance with respect to chemical action ; but the principal facts 

 have been already adverted to, under other heads, and will be con- 

 stantly illustrated in our whole progress through the science of chem- 

 istry. In general, however, it may be said that there are few chemi- 

 cal events which are not either brought on by change of tempera- 

 ture, or which do not induce a change in that particular. 



8. Pressure, is an important auxiliary to chemical action. It often 

 determines its commencement, as in the fulminating powders, and the 

 mixtures of the chlorate of potassa with combustibles. It appears to 

 operate both by causing approximation of particles, and by inducing 

 augmentation of temperature. Its agency on elastic fluids in relation 

 to each other, and in relation to them and gross fluids, and even to 

 solids, is not less important. But most of the leading facts have 

 been mentioned already, or will be mentioned hereafter. 



Probably in consequence of the heat evolved. 



