MOLECULES AND ATOMS 311 



and visible use of the discovery of the laws of nature lies in, and is 

 expressed by, the fact that they enable the unknown to be foretold, and 

 the unobserved to be foreseen. The arrangement of the experiment is 

 based on the following reasoning. 14 According to the law and to 

 experiment the density of ammonia, NH 3 , is 8J, and of hydrochloric 

 acid, HC1, 18|. 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 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 discovered by means of moist red litmus paper, which 

 should 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 substance passing 

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

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

 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 in the central portion of a glass tube. 15 The asbestos 

 forms a porous mass, which is not changed 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's 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 corresponding litmus turns 

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

 the sal-ammoniac is heated, therefore 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. 



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



M 



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



14 The beginner must remember that an experiment and the mode in which it is 

 carried out must be determined by the principle or fact which it is intended to illustrate, 

 and not vice versa as some suppose. The idea which determines the necessity of an 

 experiment is the chief consideration. 



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. 



