818 PRINCIPLES OF CHEMISTRY 



Acid, HC1, 18J, if the density of hydrogen=l. Consequently, in a 

 mixture of NH 3 and HC1, the ammonia will penetrate much more 

 rapidly through a porous mass, or a fine orifice, than the heavier hydro- 

 chloric acid, just as in a former experiment the hydrogen penetrated 

 more rapidly than the oxygen. Therefore, if the vapour of sal-ammoniac 

 comes into contact with a porous mass, the ammonia will pass through 

 it in greater quantities than the hydrochloric acid, and this excess of 

 ammonia may be detected by means of moist red litmus paper, -which 

 shpuld be turned blue. If the vapour of sal-ammoniac were not 

 decomposed, it would pass through the porous mass as a whole, and the 

 colour of the litmus paper would not be altered, because sal-ammoniac 

 is a neutral salt. Thus, by testing with litmus the substances passing 

 through the porous mass, it may be decided whether the sal-ammoniao 

 is decomposed or not when passing into vapour. Sal-ammoniao 

 volatilises at so moderate a temperature that the experiment may be 

 conducted in a glass tube heated by means of a lamp, an asbestos 

 plug being placed near the centre of the tube. 15 The asbestos forms 

 a porous mass, which is unaltered at a high temperature. A piece 

 of dry sal-ammoniac is placed at one side of the asbestos plug, 

 and is heated by a Bunsen burner. The vapours formed are driven 

 by a current of air forced from a gasometer or bag through two tubes 

 containing pieces of moist litmus paper, one blue and one red paper in 

 each. If the sal-ammoniac be heated, then the ammonia appears on 

 the opposite side of the asbestos plug, and the litmus there turns 

 blue. And as an excess of hydrochloric acid remains on the side where 

 the sal-ammoniac is heated, it turns the litmus at that end red. 

 This proves that the sal-ammoniac, when converted into vapour, 

 splits up into ammonia and hydrochloric acid, and at the same time 

 gives an instance of the possibility of correctly conjecturing a fact on 

 the basis of the law of Avogadro-Gerhardt. 15 bis 



So also the fact of a decomposition may be proved in the other 



instances where ^- proved greater than 2, and hence the apparent 



deviations appear in reality as an excellent proof of the general applica* 

 tion and significance of the law of Avogadro-Gerhardt. 



15 It is important that the tubes, asbestos, and sal-ammoniac should be dry, as other* 

 wise the moisture retains the ammonia and hydrogen chloride. 



I* bis Baker (1894) showed that the decomposition of NH 4 C1 in the act of volatilising 

 only takes place in the presence of water, traces of which are amply sufficient, but that 

 in the total absence of moisture (attained by carefully drying with PjOs) there is no 

 decomposition, and the vapour density of the sal-ammoniac is found to be normal, .i.e., 

 nearly 27. It is not yet quite clear what part he trace of moisture playfc here, and it 

 must be presumed that the phenomenon belongs to the category of electrical and contact 

 phenomena, which have not yet been fully explained (see Chapter IX., Note 29). 



