Chemistry and Physics. 483 
theory which Mr. Crookes has so fully worked out. One of th 
most remarkable of these effects is that produced by the viscosity 
of air which at a rarefaction of 129 millionths of an atmosphere, 
appears to be only a little less than its viscosity at the normal 
density, and hence the vanes of a radiometer at a speed of 100 
revolutions a minute exert a considerable drag on a disk of mica 
ae just above them 
reading these very interesting papers one is greatly im- 
pressed by the perfection to which the mercury pump, and the 
ans of measuring the degree of exhaustion which the pump 
produces, has been brought. In some of his experiments Mr. 
rookes speaks of carrying the exhaustion to four cvanilliels ths 
of an atmosphere, and we quote as follows from the close of his 
“Tn con cluding zea abstract of my researches on Repulsion 
resulting from Radiation I cannot Severe from pointing out how 
8 0 vacuum’ are 
one thousand times was said to produce a vacuum. Later a ‘ per- 
fect vacuum’ was said to be produced by chemical absorption, and 
y the Sprengel pump, the test being that electricity would not 
pass; this point being reached when the air is rarefied one hun- 
dred thousand times. Now Mr. Johnston Stony has calculated 
that the number of molecules in a cubic centimeter of air at the 
h 
are usually circular, from three to twelve inoheat in diameter, made 
of bronze, and with a bronze handle covered with bamboo. The 
reflecting face is generally more or less convex, polished with a 
~ re —— and the back is ornamented with a varied de- 
¢ property, which is only possessed by a few rare 
ioe cimens, iponiel when a bright beam of light is reflected by the 
Sait surtiee e on to a screen. The re is then seen on the screen 
an i 
vorticity cine Ase the pattern on she bac k of the mirror, 
though not only is the latter wholly hidden from the light bo 
also the pol lished surface itself, if looked at directly, acts like an 
rdinary mirror reflecting ‘ee objects i in — of it, and giving no 
indications whatever of the raised pattern n the back. In the 
oe evening discourse at the Royal Institution” of Jan. 24, 
reported in Nature of April 10th, Prof. W. E. Ayrton gives a full 
