PHYSICAL EXPFRIMENTS. 45 



blown thin, and projected in the diverging beam (Fig. 

 21). The bubble will be sharply defined upon the 

 screen, and its magnitude will depend upon the diverg- 

 ence of the beam of light, and its distance from the 

 screen. It may be made ten or fifteen feet in diameter, 

 if the lens have a short focus. The colors will begin to 

 appear around the pipe in bands, and computation of 

 the thickness may be made, and of the probable num- 

 ber of molecules in its thickness. For the considera- 

 tion of this, see " The New Chemistry," by Professor 

 Cooke, and Nature, Vol. I, p. 551; also Galloway's 

 "First Steps in Chemistry," article 102. 



POROSITY. 



The gases dissolved in common water will be ex- 

 pelled by gently heating some in a test-tube while the 

 whole is projected. The bubbles will be seen to form 

 and rise where nothing was before visible. The po- 

 rosity of water can be shown by projecting a test-tube 

 half filled with it, and its depth marked by a bit of 

 thread tied about the tube at the level of the surface. 

 A considerable quantity of salt or sugar can be added 

 to the water without noticeably increasing its bulk. A 

 piece of chalk dropped into a test-tube containing 

 warm water will at once give out quite a quantity of 

 included air. 



The ordinary experiment of showing the porosity of 

 leather by forcing mercury through it by atmospheric 

 pressure into a partial vacuum, can be exhibited by pro- 

 jecting the upper part of the tube, while the exhaus- 

 tion is going on. The mercury will be seen to fall 

 upward on account of the inverting by the lens. 



A mixture of equal parts of strong sulphuric acid 

 and water loses notably in volume when cool. Fill a 



