AZOTIC GAS. 107 



is a difficult process, the weight, instead of be- 

 ing 19-375 grains, will only be 18-25 grains. 



From these two experiments, we are entitled 

 to affirm, that the atomic weight of nitric acid is 

 6-75. 



4. To demonstrate the true composition of4 nal y sisof 



1 nitre 



nitric acid, I shall employ nitrate of potash. 



This salt, when pure, contains no water of 

 crystallization ; but a little water is often me- 

 chanically lodged between the plates of the crys- 

 tals. This water is easily dissipated by keeping 

 the salt for some time in a moderate heat ; or 

 simply, by bringing it into fusion. The salt 

 upon which the following experiments were made 

 was pure, and perfectly anhydrous. 



There is a particular temperature, higher than By heat; 

 600, but I do not know how much, in which, if 

 saltpetre be kept, it allows a certain portion of 

 oxygen gas to escape and no more. When 127J- 

 grains of nitre are kept in this temperature dur- 

 ing the requisite time, there are extricated 29 \ 

 cubic inches of pure oxygen gas ; the weight of 

 the nitre is reduced to 117i grains, and the salt 

 is no longer nitrate of potash, but nitrite of pot- 

 ash ; for when acetic acid is poured on it, red 

 fumes are extricated, which is not the case with 

 saltpetre. Now, it will be shown in a subsequent 

 part of this chapter, that the atomic weight of ni- 

 trous acid (the acid in nitrite of potash) is 5*75. It 

 will appear immediately, that nitric acid is com- 



