with a modified form of Sprengel-pump. 93 
to which the gas is exposed during measurement. By employ- 
ing a cylinder filled with mercury instead of the usual caoutch- 
ouc tubing small bubbles of air are prevented from entering 
the gauge along with the mercury. An adjustable brace or 
support is used which prevents accident to the cylinder when 
the pump is inclined for the purpose of pumping out the 
vacuum-bulb, The maximum pressure that can be employed 
in the gauge used by me is 100™™. 
the tubing of the pump is supported at a distance of 
about 55™= from the wood-work ; this is effected by the use 
of simple adjustable supports and adjustable clamps; the lat- 
ter have proved a great convenience. The object is to gain 
the ability to heat with a Bunsen burner all parts of the pump 
without burning the wood-work. Where glass and wood nec- 
essarily come in contact the wood is protected by metal or 
simply painted with a saturated solution of alum. The glass 
portions of the pump I have contrived to anneal completely 
by the simple means mentioned below. If the glass is not an- 
nealed it is certain to crack when subjected to heat, thus caus- 
ing vexation and loss of time. The mercury was purified by 
the same method that was used by W. Siemens (Pogg. Anna- 
len, vol. cx, p. 20), that is, by a little strong sulphuric acid to 
which a few drops of nitric acid had been added ; it was dried 
by pouring it repeatedly from one hot dry vessel to another, 
by filtering it while quite warm, the drying being completed 
finally by the action of the pump itself. All the measure- 
ments were made bya fine cathetometer which was constructed 
for me by William Grunow; see this Journal, Jan., 1874, p. 
23. It was provided with a well-corrected object-glass having 
a focal length of 200™", and as used by me gave a magnify- 
- Ing power of 16 diameters. 
Manipulation.—The necessary connections are effected with 
a cement made by melting Burgundy pitch with three or 
four per cent of gutta percha. It is indispensable that the 
cement when cold should be so hard as completely to resist 
taking any impression from the finger nail, otherwise it is cer- 
tain to yield gradually and finally to give rise to leaks. The 
connecting tubes are selected so as to fit as closely as possible, 
and after being put into position are heated to the proper 
amount, when the edges are touched with a fragment of cold 
cement which enters by capillary attraction and forms a trans- 
parent joint that can from time to time be examined with a 
ens for the colors of thin plates, which always precede a leak. 
Joints of this kind have been in use by me for two months at 
a time without showing a trace of leakage, and the evidence 
gathered in another series of unfinished experiments goes to 
show that no appreciable amount of vapor is furnished by the - 
