270 HEAT ACTING ON COMPOUNDS. 



originally in any substance, is the result, or effect 

 of a combination which took place before we ex- 

 amined that substance; and that every change 

 which we find to take place in any substance, is 

 the result of a combination immediately preceding 

 that change. The combination may take place 

 in two ways, because it may, in the case of the 

 individual substance, be either an adding to it or 

 a taking away from it ; and the addition, or the 

 subtraction may either be that which we can ob- 

 tain and examine in a separate state, or it may 

 not. It may happen, also, that those two modes 

 of change are combined, and' the combinations of 

 them may be varied, it may consist of any two 

 of them, or of any three, or of all the four. 



Removing a spot of tar is a familiar instance of 

 that. Common soap will not dissolve the tar, and 

 neither tar nor grease will dissolve in water ; but 

 the tar and grease combine and soften ; and, when 

 soap is added, the compound is soluble in water, 

 and the spot is removed. 



Limestone, or marble, may be heated in the 

 dry fire, till it become quick lime ; and while that 

 lime is hot in the kiln, it has an increased action 

 of heat in it; and it parts with water and with car- 

 bonic acid. After it is removed from the kiln, the 

 action of heat is communicated to the surround- 

 ing atmosphere, till that and the lime have the 

 same temperature ; and if the atmosphere is moist 

 enough, the lime takes water back again out of 

 that. So also some vegetable substances, (such 

 as the Jatropha Manihot, of the tuberous roots of 

 which the Indians of central America make their 

 Cassava bread, and which in its raw state is a poi- 

 son) may, by moist heat have both the water and 



