EDWAKD DIVERS 17 



a little iiitrio oxide is in excess of the nitric peroxide, and 

 therefore is passing unabsorbed, as a guarantee that the latter 

 does not act upon the solution in absence of its equivalent of 

 the former and thus produce some nitrate. To free the gases 

 from volatilising nitric acid they may be passed through a bot- 

 tle or tube, either empty or packed loosely with cotton. The 

 finished solution must be almost neutral ; if [acid,^ it is to be 

 boiled till neutral, before exposing it to the air. A concen- 

 trated solution of alkali nitrite dissolves a little nitrous acid 

 without decomposing it, as water alone would. To get the salt 

 in the solid state or to crystallise out the sodium nitrite, where 

 it is necessary to be sure of absence of all nitrate in it, the 

 solution may be freely evaporated, even at a boiling heat, with- 

 out decomposing or oxidising it. 



The alkali nitrites have been very imperfectly described, 

 and need examination. In the mean time, some points in their 

 description are here given. Sodium nitrite and potassium nitrite 

 are distinctly though faintly yellow and give markedly yellow 

 solutions in a little water. They are very slightly alkaline to 

 litmus. At 15° sodium nitrite, 5 parts, requires water, 6 parts, 

 to dissolve it. Potassium nitrite is soluble in about one-third 

 of its weight of water. Sodium nitrite melts at 271° (mercury 

 thread immersed). Sodium nitrite is moderately deliquescent, 

 remaining dry in winter-cold weather. Potassium nitrite is ex- 

 ceedingly deliquescent. This nitrite is got in very small, thick, 

 prismatic crystals, but sodium nitrite crystallises in very thin, 

 flattened prisms, often very large. Sodium nitrite is well known 

 to be anhydrous ; not so, potassium nitrite, crystals of which 

 are reputed to contain half a molecule of water. I have ex- 

 amined small, but distinct and separate, crystals, taken from 



