SOLIDS FORMED BY COMBINATION OF GASES. 



195 



with concentrated hydrochloric acid solution, so that a film of the 

 fluid adheres to the whole of the inner surface (this is easily 

 effected by nearly filling the tumbler with the liquid, and pouring- 

 back as much as possible into the bottle), and the other similarly 

 with strong solution of ammonia, Hold the latter bottom upwards, 

 covering the mouth with a glass plate or card, the former being 

 similarly covered and standing on the table bottom downwards ; 

 bring the two tumblers together, one over the other (fig. 87), and 

 withdraw the two cards a and b ; the ammoniacal and acid vapours 

 will intermix and combine, filling both glasses 

 Avith thick smoky fumes, which will by and by 

 condense in solid flakes on their sides. 



The air of stables often contains a sufficient 

 amount of ammonia (from the decomposition of 

 dung and other animal matter), to cause a thick 

 fume when a little strong hydrochloric acid is 

 poured out on the ground or into a saucer; 

 similarly, the presence of ammonia is soon 

 recognised by the change of colour of a mois- 

 tened red litmus paper or other analogous test- 

 paper held in the stable for a short time (Expt. 

 142). 



A number of substances closely akin to 

 ammonia, and either gaseous at the ordinary 

 temperature, or easily converted into vapour, 

 are known to chemists, all of which possess 

 the same property as ammonia itself, viz., that 

 of combining with hydrochloric acid gas to 

 form solid compounds ; also certain other acid Fig. 

 gases are known, which will behave in the 

 same way as hydrochloric acid gas, e.g., the 

 gases termed hydrobromic acid and liydriodw add. Thus the gas 

 termed phospliine (phosphoretted hydrogen) will combine with 

 hydriodic acid gas to form solid crystals of the compound termed 

 phosplionium iodide; whilst the vapour of the highly volatile 

 compound methylamine similarly combines with hydrobromic acid 

 gas to form a solid compound analogous to salammoniac. 



87. Ammonia 

 and Hydrochlo- 

 ric Acid. 



Combination of Gases to form Gases. 



In actions of this kind there is obviously no change of physical 

 state; but it generally happens that the compound formed by 

 the union of two gases condenses to a liquid on cooling more 

 readily than either of the two original gases. Thus, when 



