426 TRANSACTIONS OF SECTION A. 
molecular weight of hydrogen (2'016), p is the measured pressure and 3 is the 
vapour density of the substance at pressure p and temperature f. 
The quartz manometers hitherto in use are voluminous and difficult to mani- 
pulate. The author has devised a quartz manometer which is exceedingly 
compact and easy to manipulate. 
It consists of a bulb of less than 1 c.c. capacity blown on a quartz tube 3 mm. 
in diameter and flattened at one end so as to form a flexible membrane 
one-tenth mm. thick. The interior of this membrane is filled with the vapour 
under investigation, while the exterior is enclosed in a quartz chamber which 
communicates with a mercury manometer. 
A distortion of the membrane caused by a difference in pressure between the 
interior and the exterior causes a small quartz plate, which is polished so as to 
act as a mirror, to undergo a rotation about an axis in the plane of the polished 
surface. 
The excursions of the image of an illuminated slit reflected from this mirror 
indicate the movements of the membrane. 
The zero point corresponding to the undistorted position of the membrane 
is indicated by a second image of the slit reflected from a fixed mirror. 
The whole manometer, with mirrors, connecting mechanism, &c., is enclosed 
in a quartz tube 8 cm. long and 1 cm. diameter. 
The author has made determinations by this method up to 1250° C. The 
method is specially adapted to the investigation of substances which dissociate 
at high temperatures. 
—— ed 
i 
