678 APPENDIX. 



atic gas is dried by passing through a tube filled with chloride 

 of calcium. 



When neutral bodies, which do not enter into definite com- 

 pounds with other substances, are volatile, as the volatile oils, Du- 

 mas has pointed out a very ingenious method of determining their 

 atomic weight by the density of their vapour. He puts into a 

 glass balloon a quantity of the substance, the density of which is 

 to be determined, and then draws out the mouth of the balloon 

 to a capillary point that it may be easily hermetically sealed. 

 The balloon is then heated from 70 to 100 above the boiling 

 point of the substance, whose specific gravity is to be determined, 

 arid it is kept at that temperature till all the excess of the sub- 

 stance is driven out of the balloon. When this has taken place 

 the capillary end of the mouth is hermetically sealed. The ves- 

 sel is now filled with vapour at a known temperature, under the 

 pressure of the atmosphere at the instant that the balloon was 

 shut. The volume of the balloon and the weight of matter con- 

 tained in it being known we have all the necessary data for de- 

 termining the specific gravity of the vapour. 



The balloon or globular glass vessel should be of clear glass, 

 equal and not too thick. Its capacity should not be less than 

 fifteen nor more than thirty cubic inches. It must be washed 

 clean in the inside, and dried by passing a current of air through 

 H while hot. The mouth must then be drawn out into a long 

 capillary tube. The air which it contains is dried by putting the 

 balloon under the receiver of an air pump, exhausting the receiv- 

 er, and causing the air to return into the receiver through a tube 

 filled with dry chloride of calcium. By repeating the exhaustion 

 two or three times the air in the balloon will be quite dry. The 

 balloon is then weighed, marking the height of the thermometer 

 and barometer at the time. 



If the substance, the density of whose vapour is to be taken, 

 acts on the air of the atmosphere, we must fill the balloon with 

 hydrogen or carbonic acid gas. 



The balloon is now to be gently heated, and the beak of it 

 plunged into the substance, the specific gravity of whose va- 

 pour is to be determined, which is supposed to be either liquid or 

 to be liquefied by a moderate heat. In proportion as the balloon 

 cools the substance enters into it. We should allow about 80 

 grains of it to enter. 



