376 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



and the rods at their ends. But the staples, being compensation 

 pieces themselves, open in the heat, and advance the weights a little 

 further in upon their own account : they assist the main compensation 

 in the heat. In the cold, however, they reduce it ; the main bar, r r, 

 bending downwards tilts out the weights, but in this case the staples 

 close a little, and bring back the weights a small portion of the way 

 ngain. Thus you get increased action in the heat, and reduced in 

 the cold, which compensates for the secondary error. The effect of 

 the main compensation is altered by raising or depressing the weights 

 upon the rods, as they then work at the extremities of longer or 

 shorter levers. 



III. DISTILLATION. 



There are two phases in the operation to which the name of dis- 

 tillation has been given an operation which is intended to separate 

 a liquid from solid matters in solution in it, or from another liquid 

 with which it is mixed. 



The first phase consists in reducing the liquid to a state of vapour 

 by boiling. If it contains foreign substances in solution, such as 

 salts, as is the case with most ordinary waters from springs, or rivers, 

 or the sea, the watery part alone is vaporized the foreign substances 

 remaining at the bottom of the vessel and their separation is thus 

 effected. If the mixture is with a liquid of another kind, boiling- 

 will still separate them, at all events partially, provided the boiling- 

 points of the different liquids is not the same, because one of the 

 liquids will rise in vapour before the other. 



Since the end proposed in both cases is to obtain in more or less 

 purity the liquid in question, it must be made to change its state 

 again after having been reduced to vapour, and to return to its primi- 

 tive condition, This is the object of the second phase of distillation, 

 and it is easily accomplished by cooling and condensing the vapour. 



Distillation is a long known industrial operation, and used to be 

 carried on by means of an apparatus known as an alembic, Fig. 259. 



This consists of a boiler a, called the cucurbit, surmounted by a 

 retort, I c, called the head. When placed on the fire and filled with 

 the water to be distilled, it communicates at d with the part of the 



