310 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
This acid is discarded altogether from the pump, and the lubrication thought at first 
to be essential to the production of a good vacuum, is now found to be unnecessary, 
provided clean, pure mercury is used. The phosphoric anhydride is contained in 
the horizontal tube f In order as far as possible to prevent the passage of mercury 
vapour, three long narrow tubes, g g, are introduced between the pump and the 
apparatus to be exhausted ; the one nearest the pump is filled with precipitated 
sulphur, the centre tube contains metallic copper reduced from its oxide, and the 
third tube phosphoric anhydride. The sulphur absorbs mercury vapour, the copper 
keeps sulphur vapour from getting into the apparatus, and the phosphoric anhydride 
is a further precaution against the introduction of aqueous vapour, the presence of 
which, in even small traces, interferes with the results. At h is a vacuum tube, 
containing aluminium wires and having a capillary bore for examining the spectra of 
the residual gas. An induction coil and battery are connected with the tube by 
wires, as shown, and in this way useful information is given as to the progress of the 
exhaustion. From the tube h two tubes branch off ; one of them, i, leads to the 
“viscosity” apparatus, and the other, j, goes to the apparatus to be exhausted. 
356. The viscosity apparatus is contained in the case h ; I do not propose to describe 
it fully now, as the researches on which it has been chiefly employed are not yet 
concluded, and detailed description will be more appropriate hereafter. I may how- 
ever state that at the lower end of the long glass tube, l, is a bulb. In this bulb 
is suspended, by a fine torsion fibre of glass, an oblong plate of mica, lampblacked 
at one end. The connexion between the bulb-stem and the pump being made by 
a long, thin, glass spiral, ah angular movement can be given to the bulb without diffi- 
culty. To the mica plate a mirror is attached, so that by means of a lamp, to, a spot 
of light is reflected to the scale, n, where its passage along the graduations gives an 
accurate representation of the movement of the mica plate inside Jc. A handle with a 
stop at each side, o, allows the whole vessel to be rotated on pivots at the top and 
bottom, through a small arc, and the observation consists in noting the successive 
amplitudes of vibration when the swing of the mica plate is started by this rotation. 
The amplitudes are observed by the passage of the index spot of light across the 
graduated scale, and they form a decreasing series with a regular logarithmic decre- 
ment. This logarithmic decrement is a constant, which may be taken as defining the 
viscosity of the gas in which the mica-plate swings. Measured in this way, the vis- 
cosity of air is represented by 0*126 at the normal pressure of the atmosphere; 
and at an exhaustion of 0*19 millims. of mercury, or 250 millionths of an atmosphere, 
it has only diminished to 0'112. After this it begins to fall off: at 200 millionths 
it is 0*110 ; at 100 millionths it is 0'096 ; at 50 millionths it is 0 - 078 ; at 20 mil- 
lionths it is 0*052 ; at 10 millionths it is 0*035 ; and at 0*1 millionth it has sunk to 
about 0*01. 
At present I will say no more of the results obtained with this portion of the appa- 
ratus, as the investigation of the decrease of viscosity in various gases and vapours 
