558 



NATURE 



\_April lo, 1879 



the wire, as in previous experiments, the blacked mica 

 now becomes the driving surface. 



The whole of this complicated arrangement of appa- 

 ratus is connected together by actual fusion of the glass 

 tubes one to another ; no joint whatever occurs in any 

 part, and a certain point of exhaustion being once 

 attained, I can leave the apparatus to itself with the 

 certainty that no leakage from without can occur. 



I take an observation with this apparatus as follows : 

 Arriving at a point when a depression of the contact-key 

 tells me by the behaviour of the rotating disks that a 

 useful observation can be taken, the pressure is first 

 measured in the McLeod apparatus. The viscosity of 

 the gas is then observed, and next the repulsion exerted 

 on the viscosity-plate by the candle. At a very high 

 exhaustion the appearance of the induction in the tube h 

 is also noted, together with the spectrum given by it. 

 The strength of the current being first regulated by the 

 resistances %>, the key, w, is pressed down, and the 

 <lirection and speed of the vanes and disk in s are taken 



by a chronograph recording to tenths of a second. Fre- 

 quently duplicate or triplicate observations are taken at 

 each pressure, time being allowed to elapse between the 

 observations for the apparatus to become cool. 



With this apparatus observations can therefore be taken 

 at each pressure, on the velocity of rotation of the disk 

 and vanes, the viscosity of the residual gas, the repulsion 

 exerted by a standard candle on a black mica plate, and 

 the appearance of an inductive spark through a tube 

 furnished with platinum wire. 



In Fig. 16 I have plotted down the observations taken in 

 air-racua from some of the data I have obtained. These 

 observations are connected together by Unes forming 

 curves ; in the curve representing the " candle repul- 

 sion," I have interpolated a few observations from other 

 experiments to fill up a gap between 59 millionths and 14 

 millionths, and to give a better idea of the direction the 

 true curve would take. The candle repulsion rises to a 

 maximum somewhere between 59 and 14 millionths of an 

 atmosphere, and then rapidly sinks up to the highest 



Fr;. 16. 



exhaustion obtained. Simultaneously the viscosity drops 

 rapidly at the high exhaustions. 



When, instead of the feeble intensity of radiation 

 which can penetrate glass from a candle some inches off, 

 I substitute the intense energy of a red-hot platinum wire 

 a few millimetres off, a steady increase of speed from 

 67 revolutions a minute at 59 millionths, 150 revolutions 

 at 14 millionths, 600 revolutions at 1 1 millionths, up to 

 over 1,000 revolutions at 6 millionths, and still increasing 

 speeds at 2 millionths and at 0*4 millionth. At an exhaus- 

 tion, where the repulsion set up by the candle is least, 

 that caused by the hot wire is greatest. 



In air, at still higher exhaustions, I could detect no 

 falling off of speed, but in a series of observations with 

 hydrogen I thought there was a diminution of velocity 

 after i millionth of an atmosphere had been reached. 



In concluding this abstract of my researches on Repul- 

 sion resulting from Radiation, I cannot refrain from 

 pointing out how erroneous the ordinary ideas of a 

 " vacuum ' ' are. Formerly an air-pump which would 

 diminish the volume of air in the receiver 1,000 times was 



said to produce a vacuum. Later a " perfect vacuum ' 

 was said to be produced by chemical absorption and by 

 the Sprengel pump, the test being that electricity would 

 not pass, this point being reached, when the air is rare- 

 fied 100,000 times. Now Mr. Johnstone Stoney has cal- 

 culated that the number of molecules in a cubic centi- 

 metre of air at the ordinary pressure is probably some- 

 thing like one thousand trillions. When this number is 

 divided by 2,500,000, there are still four hundred billion 

 molecules in every cubic centimetre of gas at the highest 

 exhaustion to which I carried the experiment, illustrated 

 in Fig. 16 — a rarefaction which would correspond to the 

 density of the atmosphere about seventy- five miles above 

 the earth's surface, that is, if its density decreases in 

 geometrical progression, as its height increases iri arith- 

 metical progression. Four hundred billion molecules in 

 a cubic centimetre appear a sufficiently large number to 

 justify the supposition that when set into vibration by a 

 white-hot wire they may be capable of exerting an 

 enormous mechanical effect. __ 



W. Crookes 



