156 THE ART OF PROJECTING. 



through the wire. If the current is sufficiently great 

 the wire coil will be heated at once, and a convection 

 current will at once show itself in the water, the heated 

 water next to the wire rising rapidly to the top. The 

 effect will be still more marked if a drop or two of 

 some one of the aniline dyes be let fall from the surface 

 over the wire. Its greater density will carry it at once 

 to the bottom ; but when the current is sent through 

 the wire, the movements in the water will be rendered 

 very plain. The bichloride of tin or the sulphate of 

 zinc will also answer the same purpose. 



CHEMISTRY. 



Most of the chemical reactions that are usually ex- 

 hibited before classes in the recitation or lecture-room 

 can be shown in a much more satisfactory way by 

 means of the apparatus for projection than in the ordi- 

 nary way. The method is moreover both cheaper and 

 easier; cheaper, because each experiment requires but 

 a few drops of the substance in a test-tube or the 

 tank, instead of the large quantity necessary for many 

 to see at once, and easier, because the preparation 

 needed for experiments upon an extended scale is 

 always tedious and tiresome. One who uses the tank 

 (Fig. 20) for the first time for projection, say of sil- 

 ver, in a solution as dilute as two or three drops of the 

 nitrate to the tank full of water, will be surprised at 

 the prodigious precipitation brought about by the addi- 

 tion of a single drop of hydrochloric acid introduced 

 upon the end of a glass rod. Great heavy clouds roll 

 and tumble about upon the screen, looking as though 

 they might weigh tons. 



