SOLIDS, LIQUIDS, AND OASES. 379 



the heat, which would otherwise convert it into olive-oil 

 vapor, begins to dissociate its constituents, and if the tem- 

 peratutre be raised a little higher, we obtain some gases, 

 but these are the products of decomposition, not gaseous 

 olive oil. This is called " destructive" distillation. 



In olive oil, the boiling-point and dissociation point are 

 near to each other. In the case of glycerine, these points 

 so nearly approximate that, although we cannot distil it un- 

 broken under ordinary atmospheric pressure, we may do so 

 if some of this pressure is removed. Under such diminished 

 pressure, the boiling-point is brought down below the dis- 

 sociation point, and condensible glycerine gas comes over 

 without decomposition. 



Sugar affords a very interesting example of dissociation, 

 commencing far below the boiling-point, and going on 

 gradually and visibly, with increasing rapidity as the tem- 

 perature is raised. Put some white sugar into a spoon, and 

 neat the spoon gradually over the smokeless gas-flame or 

 spirit-lamp. At first the sugar melts, then becomes yellow 

 (barley sugar); this color deepens to orange, then red, then 

 chestnut-brown, then dark brown, then nearly black (cara- 

 mel), then quite black, and finally it becomes a mere cinder. 

 Sugar is composed of carbon and water; the heat dissociates 

 this compound, separates the water, which passes off as 

 vapor, and leaves the carbon behind. The gradual deepen- 

 ing of the color indicates the gradual carbonization, which 

 is completed when only the dry insoluble cinder remains. 

 An appearance of boiling is seen, but this is the boiling of 

 the dissociated water, not of the sugar. 



The dissociation temperature of water is far above its 

 boiling-point. It is 5072 Fahr., under conditions corre- 

 sponding to those which make its boiling-point 212. If 

 we examine the variations of the boiling-point of water, as 

 the atmospheric pressure on its surface varies, some curious 

 results follow. To do this the reader must endure some 

 figures. They are extremely simple, and perfectly intelli- 

 gible, but demand just a little attention. 



Following are three columns of figures. The first repre- 

 sents atmospheres of pressure i.e., taking our atmospheric 

 pressure when it supports 30 inches of mercury in the bar- 



