1876.] Mr. AY. Crookes on the Radiometer. 307 



difficult to exhaust a radiometer to such a point that it will not move to 

 a candle placed a few inches off; but I have not yet succeeded in 

 stopping the moyement of the beam in the torsion-apparatus. 



A long series of observations have been taken, at different degrees of 

 exhaustion, on the conductivity of the residual gas to the spark from an 

 induction-coil. Working with, air, I find that at a pressure of about 40 

 millionths of an atmosphere, when the repulsive force is near its maxi- 

 mum, a spark, whose striking distance at the normal pressure is half an 

 inch, will illuminate a tube having aluminium terminals 3 millimetres 

 apart. When I push the exhaustion further, the j-inch spark ceases to 

 pass ; but a 1-inch spark will still illuminate the tube. As I get nearer 

 to a vacuum more power is requu-ed to drive the spark through the tube ; 

 but at the highest exhaustions I can still get indications of conductiA'ity 

 when an induction-coil actuated \\ith five Grrove's cells, and capable 

 of giving a 6-inch spark, is used. 



When so powerful a spark is employed there is great danger of per- 

 forating the glass, thus causing a very shght leakage of air into the ap- 

 paratus. The log. dec. now slowly rises, the repulsive force of the candle 

 increases to its maximum, and then slowly diminishes to zero, the log. 

 dec. continuing to rise till it shows that the internal and external pres- 

 sures are identical. With a fine perforation several days are occu- 

 pied in going through these phases, and they take place with such slow- 

 ness and regularity as to afford opportunities for getting valuable 

 observations. 



The improvements now added by Mr. Gimingham to the pump render 

 it so easy to obtain high exhaustions that, in preparing experimental 

 radiometers, I prefer to exhaust direct to one or two milhonths of an 

 atmosphere. By keeping the apparatus during this exhaustion in a hot- 

 air bath heated to about 300° C. for some hours, the occluded gases are 

 driven off from the interior surface of the glass and the fiy of the radio- 

 meter. The whole is then allowed to cool, and attenuated air from the 

 air-trap is put in in small quantities at a time, until the McLeod gauge 

 shows that the best exhaustion for sensitiveness is reached ; if necessary, 

 this point is also ascertained by testing with a candle. Working in this 

 way, I can now do in a few hours what formerly required as many days. 

 In this manner, emploviug hydrogen instead of air for the gaseous resi- 

 due, and using roasted mica vanes set at an angle with the axis, as 

 described further on, I can get very considerably increased sensitiveness 

 in radiometers. I am still unable, however, to get them to move in 

 moonhght. The statements made by an observer nearly a year ago, that 

 he obtained strong rotation by moonlight, must therefore be considered 

 erroneous. My most sensitive torsion-balance will, however, move easily 

 to moonlight. 



The above-mentioned facts, in addition to what has already been pub- 



