1912-13.] Determining Vapour Densities at High Temperatures. 3 
move outwards or inwards. The quartz rod which connects the plate d 
with the tube b is constricted at e, so that it is flexible at this point. 
The constriction is made as close to the plate d as possible, so that 
the angular displacement of the plate for a given displacement -of the 
membrane may be as great as possible. Both surfaces of d are dull so 
that they do not reflect. A small quartz plate / is rigidly connected with 
d. The inner surface of / (nearest the membrane) is dull, while the outer 
surface is polished so as to act as a mirror. A third quartz plate g, exactly 
similar to /, is rigidly connected with the tube b, and adjusted so that its 
polished surface is as nearly as possible in the same plane with the polished 
surface of /. 
Fig. 3 shows the method of using the manometer for vapour density 
determinations. The main hulk of the vapour is contained in the bulb A. 
E and F are sealed off at D before the determination. A is connected with 
the manometer by means of a long capillary tube B. A series of small 
tubes C permit of the removal of small portions of the substance under 
investigation, without the necessity of opening the apparatus. A portion 
of the substance is distilled or sublimed into the outermost end of the 
last tube, which is then sealed off at the constriction. This is a great 
saving of trouble when it is desired to make a series of determinations 
at varying concentrations, as for example in the investigation of a dis- 
sociating gas. 
The manometer is enclosed in a quartz jacket G which is closed at 
one end by a plane disk H of polished quartz. The tube K leads to the 
mercury manometer and to the regulating apparatus, a sketch of which 
is shown in fig. 4. 
In order to determine the pressure of the gas in A, the pressure of 
the air inside the quartz jacket G is regulated until the movable mirror 
/ (fig. 2) is in the zero position, i.e. in the position which it occupies when 
the pressure in A is equal to the pressure in G. The pressure is then read 
off on the mercury manometer g (fig. 4). 
The deflection of the mirror is magnified and observed by the method 
which is employed in the Thomson mirror galvanometer. 
An illuminated slit, or Nernst filament, and a convex lens of about 
