98 



ELEMENTS OF LABORATORY WORK 



of the solid, for some time at a temperature exceeding the 

 temperature to be investigated. It may, for example, be 

 boiled for several minutes. A thermometer is then inserted, 

 and, as soon as the liquid has cooled to the desired tempera- 

 ture, the same quantity of the solution is transferred by a 

 warm pipette to a weighed crucible, evaporated, and measured 

 as before. 



It follows from this that a saturated solution, as it cools, 

 must liberate some of the solid from solution. This may be 

 seen by allowing some of the warm solution to cool in a clean 

 vessel. The solid then appears as crystals, first minute, then 

 growing larger and increasing in number. 



By this and other methods such diagrams 

 as that of fig. 31 have been drawn up. The 

 curves represent the relative solubilities of 

 various substances at various temperatures. 



66. When Gases differing in Composition 

 are placed in Contact, a gradual Rear- 

 rangement of the Matter proceeds until 

 the whole is homogeneous, i.e. until the 

 Composition is the same in every Part. 

 In the same way as certain liquids diffuse 

 throughout each other, so do gases ; but the 

 diffusion is completed, in the case of gases, 

 more rapidly, even when the surfaces in con- 

 tact are very small. The manipulation of a 

 gas is more difficult than that of a liquid, 

 and direct proofs of the diffusion of gases are 

 scarcely needed in the presence of many 

 indirect proofs, but the following experiment 

 illustrates the diffusion of two gases into one 

 another : 



Two dry glass flasks (A and B) of equal 

 capacity, each fitted with a cork and glass 

 Fig. 32. j^g to which a piece of caoutchouc tube 



with a clip is added, and containing air and ammonia respec- 

 tively, are placed in communication by a narrow tube, and the 

 clips opened so that the two gases may come into contact. 



