302 SCIENCE IN SHORT CHAPTERS. 



in the condition of solid or liquid, and are more easily broken by the 

 expansive energy of heat. 



To illustrate this, let us take two common and well-known oils, 

 olive oil and turpentine. The first belongs to the class of " fixed 

 oils," the second to the " volatile oils." If we apply heat to liquid 

 turpentine, it boils, passes into the state of gaseous turpentine, 

 which is easily condensible by cooling it. If the liquid result of this 

 condensation is examined, we find it to be turpentine as before. 

 Not so with the olive oil. Just as this reaches its boiling-point, the 

 heat, which would otherwise convert it into olive-oil vapor, begins 

 to dissociate its constituents, and if the temperature be raised a little 

 higher, we obtain some gases, but these are the products of decompo- 

 sition, not gaseous olive oil. This is called " destructive' ' distilla- 

 tion. 



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

 each other. In the case of glycerine, these points so nearly approxi- 

 mate that, although we cannot distil it unbroken 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 dissociation point, and condensible glycer- 

 ine gas comes over without decomposition. 



Sugar affords a very interesting example of dissociation, commenc- 

 ing far below the boiling point, and going on gradually and visibly, 

 with increasing rapidity as the temperature is raised. Put some 

 white sugar into a spoon, and heat the spoon gradually over a smoke- 

 less 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 neary black (caramel), then 

 quite black, and finally it becomes a mere cinder. Sugar is com- 

 posed of carbon and water ; the heat dissociates this compound, sep- 

 arates the water, which passes off as vapor, and leaves the carbon be- 

 hind. The gradual deepening of the color indicates the gradual car- 

 bonization, 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 corresponding 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 in- 

 telligible, but demand just a little attention. 



Following are three columns of figures. The first represents 

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

 it supports 30 inches of mercury in the barometer tube as a unit, 

 that pressure is doubled, trebled, etc. up to twenty times in the first 

 column. The second column states the temperature at which water 

 boils when under the different pressures thus indicated. The third 

 column, which is the subject for special study just now, shows how 

 much we must raise the temperature of the water in order to make 

 it boil as we go on adding atmospheres of pressure : or, in other 

 words, the increase of temperature due to each increase of one 

 atmosphere of pressure. The figures are founded on the experiment 

 of Kegnault. 



