65 



point it must be adjusted to it by the screw fixed on the brass 

 frame of the wedge. 



If the tube be now filled with the solution of sugar and laid 

 in its place, colour dissociation takes place, and may be made to 

 vanish again by moving the quartz wedge so that the 100 point 

 on the scale approaches the zero on the fixed vernier. If the 

 solution be perfectly colourless the screw fixed on the horizontal 

 rod turning the regulator need not be touched. If the solution 

 is coloured, the screw must be turned to the right or left until 

 the tint used in fixing the zero is obtained. By several adjust- 

 ments of the quartz wedge to the point of colour-uniformity we 

 arrive at an accurate determination of the rotation. The per- 

 centage of sugar may then be calculated from the following 

 propositions : 



(1.) The amount of deviation of the plane of polarisation 

 is proportional to the length of the liquid column in 

 the tube. 



(2.) The deviation is proportional to the concentration, i.e., to 

 the number of grammes of sugar in the unit volume 

 (100 or 1,000 c.c.) of solution. 



Accordingly by determining once for all the angle of rotation 

 given by a single saccharine solution of known concentration 

 in a tube of a certain length, we are able by simple proportion 

 to calculate the number of grammes of sugar in 100 c.c. of 

 any solution of unknown strength, from its observed angle of 

 rotation. 



See Handbook of the Polariscope (Macrnillan and Co., 

 1882), 



205. Lunge's nitrometer, graduated to 50 c.c.'s, is used 

 to estimate the amount of oxides of nitrogen in a gaseous 

 mixture or a nitrate, &c. Price IBs. 6d. 



1886. Exhibited by Toivnson fy Mercer. 



The oxides of nitrogen must first be absorbed in strong 

 sulphuric acid and then submitted to analysis in the nitrometer, 

 where the whole of the nitrogen is liberated as nitric oxide. 

 The manipulation is as follows : The level tube is raised, and the 

 burette filled with mercury until the latter enters the stopcock. 

 The tap is now closed, so that its ways communicate neither 

 with the inside nor outside of the burette, and a certain volume 

 of nitrous vitriol is poured into the cup. The level tube is now 

 lowered and the tap cautiously opened so that the acid may enter 

 the burette, and is suddenly closed when all the acid is run in, 

 except a small drop, and no air has entered. The cup is rinsed 

 and re-rinsed with a few c.c.'s of pure sulphuric and the acid 

 allowed to enter the burette, but no air. The burette is then 

 shaken, so that the acid and mercury intermix, when the gas is 

 evolved. The mercury in the two tubes is brought to same 

 level and allowance made for the acid in the burette tube, and 



