46 THE ART OF PR OJE C TING. 



test-tube with the fresh mixture, and tie a string about 

 the tube at the night of the mixture. It will be too 

 hot for handling with the fingers at first, but it may be 

 cooled in a few minutes enough to show the shrinkage, 

 by stirring it in a dish of cold water. The surface will 

 be seen to be considerably below the string which 

 marked its original hight. This experiment may be 

 used to exhibit compressibility of liquids. 



Most of the experiments which are suitable for pro- 

 jection of the properties of matter are chemical, and 

 will be found described under that head. Diagrams, 

 such as are given in most text-books on mechanics, can 

 be made upon glass by one of the processes described 

 on page 30, and will be found very convenient to a lec- 

 turer upon that subject. 



COHESION. 



A drop of water or other fluid exhibits this, and may 

 be projected with the lantern, or with the porte lumiere, 

 and a single lens (Fig. 28). Sprinkle a little lamp- 

 black or lycopodium-powder 

 upon one side of a strip of 

 glass, like a microscope slide- 

 and place it in the proper 

 place for projecting, keep- 

 ing it horizontal that the 

 dust may not slide off. 

 28 ' Now place a single drop of 



water upon the slide ; the powder will prevent it from 

 spreading upon the glass, and it will gather itself up 

 into a round globule with some of the dust over its sur- 

 face, making an interesting object upon the screen. 



Again, a saturated solution of zinc-sulphate is put into 

 a white glass square bottle, two inches square,, and 



