bakus.j VISCOSITY OF GASES. 249 
is given by an inspection of the results obtained. Any flaw in the con- 
nections can therefore be remedied only at the expense of much time. 
Differential apparatus. — Before passing to the method used for heating 
it may be well to insert a few remarks relative to the differential appa- 
ratus. This is closely analogous in character to a differential galvanom- 
eter, and the rates of transpirations through tubes, one of which is hot the 
other cold, are compared. The arrangement of this apparatus is readily 
seen in Fig. 46. A lateral arm, IP, of the tube g h communicates with 
the cold helix J', which is completely submerged in the water of the 
trough M i¥, and provided with its own graduated tube V I and water- 
jacket i i'. Except in temperature, the capillaries and their pneumatic 
appurtenances are identical in form. Certain special desiderata will 
be indicated below. 
Method of heating. — All the soldered parts of the capillary apparatus 
being thus thermally protected by a current of water from the hydrant, 
the heating of the spiral is not a difficult problem. To make observa- 
tions at the low temperature (as near 0° 0. as convenient) a current of 
water may be showered upon the helix out of the water pipes. But it 
is equally good to siphon the water out of the trough M M. For 100° 
the helix is appropriately surrounded by a non-conducting tube, through 
which steam circulates freely. Admirably constant mean temperatures 
are obtained as high as 1,000° by simply heating the helix in a chimneyed 
Bunsen burner. For 1,300° the Bunsen burner is replaced by a blast- 
lamp fed by a regular current of air. In the case of these high tem- 
peratures the helix is surrounded by a cylindrical tube of asbestus, as 
shown at n n, Figs. 45 and 46. These cylinders are exceedingly con- 
venient and may be made by soaking asbestus board in water and roll- 
ing it around a cylindrical stick of suitable diameter. After drying, 
the cylinders are ready for use. Parts liable to breakage are, of course, 
protected from radiation by asbestus screens. Such subsidiary screen- 
ing is everywhere necessary, and need not be described here. Great 
difficulties were encountered in endeavoring to obtain satisfactorily con- 
stant temperatures between 300° and 1,000°. After much vain search- 
ing I finally tried an ordinary oil student lamp for the purpose, and ob- 
tained excellent results. The space in the chimney of such a lamp above 
the flame is available for a hot-air bath. In Fig. 45, n' n' shows the 
position of the chimney of this lamp. The temperatures thus obtain- 
able, besides being constant or of very slow regular variations (increase) 
(903) 
