52 CHEMISTRY. 



Fig. 4. 



the time, however, that it arrives at the end of the tube it 

 is so far cooled as to become liquid, and drops into the 

 receiver, b. But the gas moves on through the tube, c, and 

 goes up the glass jar, forcing the water in it down as fast 

 as it collects. 



50. Difference between Gases and Vapors. You see in 

 the above process what the difference is between a gas and 

 a vapor. The mercury rises in vapor with the oxygen 

 gas, and both are invisible as they pass mingled together 

 through the tube of the retort. This is because the parti- 

 cles of the mercury are so much separated from each other 

 by the action of heat, just as it is with water when it is 

 converted into steam. But these particles are condensed 

 or brought near together again, and appear in the liquid 

 form in the receiver, b. Meanwhile the gas, though cooled 

 equally with the mercury, retains its gaseous form, and 

 passes on. You see that the vapor has its gaseous form 

 dependent upon a certain range of temperature, but the gas 

 retains it under all temperatures. No degree of cold (that 

 is, diminution of heat) can condense the gas into a liquid 

 form. The form of the gas, then, is not accidental and 

 temporary, but permanent. The range of temperature in 

 which vaporization can take place is different in different 

 substances. Water can evaporate at all temperatures, while 



