268 PRINCIPLES OF CHEMISTRY 



well as oxygen in unstable combination with nitrogen, in the form 

 of the radicle NO 2 of nitric acid, the nitre-compounds, when ignited or 



common writing-paper, and in flax, &c. ; under the action of nitric acid it forms water and 

 nitrocellulose, which, although it has the same appearance as the cotton originally taken, 

 differs from it entirely in properties. It explodes when struck, bursts into flame very 

 easily under the action of sparks, and acts like gunpowder, whence its name of pyroxy- 

 lin, or gun-cotton. The composition of gun-cotton is C 6 H 7 N 3 O U = C 6 H 10 O 5 -f 8NHO 3 

 3HoO. The proportion of the group NO 2 in nitrocellulose may be decreased by limiting 

 the action of the nitric acid, and a compound is obtained which burns without explosion, 

 although it is capable of bursting into flame. This substance when dissolved in a mix- 

 ture of alcohol and ether is called collodion. The solution when poured on to any 

 surface loses all the ether and alcohol by evaporation, and leaves an amorphous mass in 

 the form of a transparent membrane insoluble in water. A solution of collodion is em- 

 ployed in medicine for covering wounds, and in wet-plate photography for giving on glass 

 an even coat of a substance into which the various reagents employed in the process are 

 introduced. 



The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, 

 and the other nitro-compounds, of burning with an explosion depends on the reasons 

 in virtue of which a mixture of nitre and charcoal deflagrates and explodes ; in both 

 cases the elements of the nitric acid occurring in the compound are decomposed, 

 the oxygen in burning unites with the carbon, and the nitrogen is set free ; thus a very 

 large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed 

 from the solid substances originally taken. These gases occupy an incomparably larger 

 volume than the original substance, and therefore produce a powerful pressure and 

 explosion. It is evident that in exploding with the development of heat (that is, in 

 decomposing, not with the absorption of energy, as is generally the case, but with the 

 evolution of energy) the nitro-compounds form stores of energy which are easily set free, 

 and that consequently their elements occur in a state of particularly energetic move- 

 ment, which is especially strong in the group NO 2 ; this group is common to all nitro- 

 compounds, and all the oxygen compounds of nitrogen are unstable, easily decom- 

 posable, and (Note 29) absorb heat in their formation. On the other hand, the nitro- 

 compounds are instructive as an example and proof of the fact that the elements and 

 groups forming compounds are united in definite order in the molecules of a com- 

 pound. A blow, concussion, or rise of temperature is necessary to bring the com- 

 bustible elements C and H into the most intimate contact with NO 2 , and to distribute 

 the elements in a new order in new compounds. 



As regards the composition of the nitro-compounds, it will be seen that the hydrogen 

 of a given substance is replaced by the complex group NO., of the nitric acid. The same 

 is observed in the passage of alkalis into nitrates, so that the reactions of substitution of 

 nitric acid that is, the formation of salts and nitro-compounds may be expressed in 

 the following manner. In these cases the hydrogen is replaced by the so-called radicle 

 of nitric acid NOo, as is evident from the following table : 



1 Caustic potash . . . KHO. 



I Nitre K(NO 2 )O. 



/Hydrate of lime . . CaH 2 O 2 . 



1 Calcium nitrate . . . Ca(NO 2 ) 2 O 2 . 



(Glycerin C 3 H 5 H 5 O 3 . 



I Nitroglycerin . . . C 3 H 5 (NO 2 ) 3 O 3 . 



(Phenol C 6 H 5 OH. 



I Picric acid .... C e H 2 (NO 2 ) 3 OH, &c. 



The difference between the salts formed by nitric acid and the nitro-compounds con- 

 sists in the fact that nitric acid is very easily separated from the salts of nitric acid by 



