370 MESSRS. V. H. VELEY AND J. J. MANLET ON 



gauge, F, introduced between the two towers served to indicate the pressure, 

 generally a few millimetres, under which the distillation was conducted, while the 

 glass springs, G and G', decreased the rigidity of the apparatus. The process of 

 distillation consisted in drawing up the acid into the flask by means of the stop- 

 cock ; the whole apparatus was then exhausted, and the water-bath surrounding 

 the distillation flask gently warmed. The nitrous fumes present in the acid were for 

 the most part immediately swept through the apparatus, and the remainder collected 

 in the first portions of the nitric acid condensed in the receiver. This was drawn off 

 and the whole process repeated until a small test sample of the acid produced no 

 decoloration of a drop of standard potassium permanganate solution after the lapse 

 of thirty minutes. The most effective method, according to our experience, for 

 removing the last trace of nitrous fumes without either diluting the acid or adding 

 any other impurity, consists in passing a stream of dried ozonised oxygen through 

 the acid, and then subsequently distilling it in the above vacuum apparatus to 

 eliminate any dissolved gas. 



DETECTION AND ESTIMATION OF IMPURITIES. 



The principal impurities likely to be present in any sample are (i.), nitrous acid, 

 formed from the decomposition of the liquid itself, or its superincumbent vapour, 

 (ii.), sulphuric acid, (iii.), the haloid acids, namely, hydrochloric and hydriodic, 

 derived from the previous manufacture of the acid from " caliche," and, (iv.), alkaline 

 silicates or silica from the glass of the containing vessels. The methods used to 

 estimate the three first-mentioned impurities are described in detail. 



(i.) Nitrous Acid. 



For this purpose the metaphenylene-diamine method was employed, by means 

 of which 1 part in 20 millions (according to GRIESS, its author) can be detected, 

 and according to our own observation a quantity less than 1 in 15, but greater than 

 1 in 20 millions is revealed. The standard test adopted was that of 1 part nitrous 

 acid in 1 million parts of water, produced either by the liberation of that amount of 

 nitrous acid by the acidification of sodium nitrite (obtained from silver nitrite) or by 

 an imitation tint formed from Bismarck-brown dissolved in dilute alcohol ; the 

 comparisons were made by the tintometer described by one of us in a previous 

 communication. 



(ii.) Halogen Acids. 



The solubility of silver chloride was fully investigated by STAS* in the course of 

 his investigations upon the atomic weights. According to this writer, the solubility 



* ' Ann. Cbim. Phys.' [4], vol. 25, p. 22, and [5], vol. 3, p. 145. 



