BY DR. LAUDER BRUNTON. 545 



one cubic centimetre shall correspond to 10 milligrammes 

 (0.010 grin.) of sodium chloride. 



This solution may be made by dissolving twenty grammes 

 of perfectly pure metallic mercury in boiling nitric acid, until 

 a drop of the acid fluid does not cause a precipitate when added 

 to a solution of common salt. The acid fluid is concentrated 

 by heating over a water-bath until it is of syrupy consistence. 

 It is then diluted with nearly a litre of distilled water. 



Unless a great excess of nitric acid has remained after the 

 evaporation, a white precipitate, consisting of a basic nitrate 

 of mercury, will fall, and must be separated by filtration. Be- 

 fore performing the latter operation, a few drops of nitric acid 

 may, however, be added, as they will cause the re-solution of a 

 considerable part of the precipitate, without rendering the 

 liquid too acid. The solution of mercuric nitrate thus made 

 must be set aside until the other reagents which are required 

 for determining its strength are prepared. 



2d. A solution made by dissolving in distilled water 20 

 grammes of pure sodium cloride and diluting to one litre. The 

 salt is fused before being weighed. 



Ten cubic centimetres of this solution contain 0.200 grm. of 

 NaCl. 



3d. A solution made by dissolving 4 grammes of pure urea 

 iu distilled water and diluting to 100 c. c. 



4th. A solution of sodium sulphate, saturated at ordinary 

 temperatures. 



In order to determine the strength of the solution of mer- 

 curic nitrate, it is poured into a burette (preferably a Mohr's 

 burette, with glass stopcock) of a capacity of 50 cubic centi- 

 metres, and divided into lOths of a cubic centimetre. 



Ten cubic centimetres of the standard solution of chloride 

 of sodium are then measured by means of a pipette, and poured 

 into a glass beaker. 



To this is added 3 cubic centimetres of the solution of urea, 

 and 5 cubic centimetres of the solution of sulphate of sodium. 

 The solution of nitrate of mercury is now allowed to flow 

 gently into the beaker : as the drops fall into the fluid con- 

 tained in the latter, a white precipitate is seen to form, which, 

 however, dissolves at once, or when the fluid is stirred. On 

 adding more of the solution of nitrate of mercury, the fluid 

 becomes opalescent but no precipitate occurs until the reaction 

 is completed, i.e., until the whole of the chloride of sodium 

 has been decomposed. 



The number of cubic centimetres of the solution of mercuric 

 nitrate which has been added is read off; if, for example, 12.7 

 cubic centimetres of the solution had to be added in order to 

 induce a permanent precipitate, we conclude that this quantity 

 of solution contains the quantity of mercuric nitrate required 

 35 



