1*22 CONTRACTION AND EXPANSION 



form aqueous vapour; which is one cause of the 

 diminished elasticity in sulphurous and sulphuric 

 acids. 



A thousand examples might be adduced equally 

 calculated to illustrate the chemical force with 

 which aethereal matter is attracted by the atoms 

 of gravitating matter, and its idio-repulsive force 

 thus diminished or destroyed. 



The same order of phenomena which marks 

 the union of sulphur and hydrogen, is exhibited 

 during the combination of phosphorus, iodine, 

 bromine, selenium, tellurium, &c. in the state of 

 vapour with hydrogen and other permanent gases. 

 The volume and elasticity of these dense vapours 

 are augmented by receiving from the lighter gases 

 a portion of the principle on which their volume, 

 &c. depend, during the very act of combining. 



The specific gravity of phosphorous vapour is 

 4-444 at the temperature of 550 F. or about 64 

 times the specific gravity of hydrogen . But when 

 it combines with hydrogen, atom to atom, it be- 

 comes highly elastic, and its particles occupy 4 

 times their former space, as in phosphuretted hy- 

 drogen, the specific gravity of which is only 17 

 times tha of hydrogen. 



The specific gravity of iodine vapour is 8' 75 

 at the temperature of 347 F. When it unites 

 with hydrogen, atom to atom, (that is, in the 

 ratio of 126 by weight to 1 of hydrogen,) hy- 

 driodic acid is formed, the specific gravity of 

 which is 4' 386, or about one-half that of iodine 



