94. O. N. Rood—Obtaining and measuring very high Vacua 
resinous compound, which, I may add, is never used until it 
has been repeatedly melted. As drying material I prefer 
caustic potash that has been in fusion just before its introduc- 
tion into the drying tube; during the process of exhaustion it 
can from time to time be heated nearly to the melting point ; 
_ if actually fused in the drying tube the latter almost invaria- 
ly cracks. The pump in the first instance is to be inclined 
at an angle of about 10 degrees, the tube of the gauge being 
supported by a semicircular piece of thick pagte-board fitted 
with two corks into the top of the cylinder. This seemingly 
awkward proceeding has in no case been attended with the 
slightest accident, and owing to the presence of the four level- 
ing-screws the pump when righted returns, as shown by the 
telescope of the cathetometer, almost exactly to its original 
place. In the inclined position the exhaustion of the vacuum- 
bulb is accomplished along with that of the rest of the pump. 
The exhaustion of the vacuum-bulb when once effected can be 
preserved to a great extent for use in future work, merely by 
allowing mercury from the reservoir to flow in a rapid stream 
at the time that air is allowed to reénter the pump. “During 
the first process of exhaustion the tube of the gauge is kept hot 
by moving to and fro a Bunsen burner, and is in this way 
freed from those portions of air and moisture that are not too 
firmly attached. After a time the vacuum bulb ceases to de- 
amount of air that was detached from the walls of the pump 
by heating them for 10 minutes somewhat above 100° C., and 
found that it was z—yyb-sy5 of the air originally present. 1 
have also noticed that a still larger amount of air is detached 
by electric discharges. This coincides with an observation of 
K. Bessel-Hagen in his interesting article on a new form of 
Topler's mercury-pump (Annalen der Physik und Chemie, 
1881, vol. xii). Even when potash is used a small amount of 
moisture always collects in fe bends of the fall-tube ; this is 
readily removed by a Bunsen-burner ; the tension of the vapor 
being greatly increased, it passes far down the fall-tube in large 
