Professor Dewar 



[Jan. 20, 



containing excess of liquid mercury, the local reduction of tempera- 

 ture produced by the evaporation soon causes a rough image of the 

 paper to appear in the form of minute globules of condensed mercury. 

 Such experiments support the view that the laws of saturated vapours 

 are maintained at very low pressures. 



In Fig. 4 specimens of the old and new vessels for collecting and 

 manipulating liquid gases are shown. In each of the old forms it 



Fig. 4. 



will be noted that a mass of phosphoric anhydride placed in the 

 lower portion is required to absorb traces of water, otherwise the 

 vessels are useless for optical observations. The vacuum receivers 

 get over this difficulty. 



The perfection of the vacuum in different vessels, all treated m 

 the same way, differs very much, and after use they almost invariably 

 deteriorate. The relative rates of evaporation of liquid oxygen under 

 the same conditions in different vessels is the best test of the vacuum. 

 In many of the large vessels used for the storage of liquid gases, it 



