76 DR. R. ANGUS SMITH ON THE EXAMINATION 



nitrites together, showing that some is oxidized and some 

 ready to become so. 



" Schonbeinhas found traces of nitrous acid in the efflo- 

 rescence of walls. In judging of the time, we must make 

 allowance for this, and not judge from small traces such 

 as he alludes to. 



^^ When nitrites and organic matter are found together, 

 they may be estimated separately. The chameleon will 

 be decomposed by both. 158 of chameleon solid is equi- 

 valent to 95 of nitrous acid, or 1000 = 606. It Avill be 

 necessary to find one of the substances separately, and to 

 subtract it from the whole, in order to find the amount of 

 the second. 



" The amount of nitrous salts may be found by using 

 ozone-paper, that is, paper with iodide of potassium and 

 starch. The water to be tried is made acid with a little 

 dilute sulphuric acid, not more than three drops to 1000, 

 and a drop put upon the paper. If nitrous acid is present, 

 it becomes blue. This blue is obtained immediately by 

 putting a drop of the solution on ozone-paper when there 

 is I of NOj in 30,000 of water; and by waiting patiently, 

 and giving time, it may be seen with much less. 



'^ In order to find how much nitrite is in water, the solu- 

 tion may be diluted until the reaction ceases to be distinct 

 even after waiting. The amount in the water will then be 

 I in 100,000. If, in another case, when we find the re- 

 action distinct in water, we take 100 grains and add 900 

 to it, and find it beginning to be indistinct, the undiluted 

 quantity must have been 10 times as strong as the di- 

 luted. The amount in the 1000 grains is i in 100,000; 

 the amount in the 100 grains is 10 times as great, or i in 

 10,000; and thus we may arrive sufficiently near to the 

 total amount. The amount of water added in order to 

 bring the reaction to the adopted minimum is the measure 

 of the strength of the solution. 



