April 8, 1^98.] 



SCIENCE. 



483 



in the latter case. In the absence of con- 

 vection currents that part of the heat 

 transmitted by the gas was probably car- 

 ried by a process analogous to conduction 

 in solids. The shortness of conductor in 

 the case of the small bulb may account for 

 the greatly increased rate of conduction. 

 But why the conductivity of a gas remains 

 nearly constant through a very wide range 

 of pressures is not clear. Mr. Crookes' ex- 

 planation of this phenomenon seems to me 

 very unsatisfactory. 



It will be noticed that the ' Ether line ' 

 is about four per cent, lower with the 

 small bulb than with the large one. This 

 may be due to the greatly decreased amount 

 of surface presented by the small bulb for 

 absorption of the radiant heat. 



The enormous heat-conducting capacity 

 of gases at very small pressures is strik- 

 ingly shown in all the curves. But hydro- 

 gen is preeminent in this respect. Thus, 

 in the large bulb, hydrogen at a pressure of 

 only twenty-six millionths of an atmos- 

 phere transmits heat as rapidly as the 

 ether. At seventy-six millionths it equals 

 air at atmospheric pressure ; that is to say, 

 it does the work of nearly two hundred 

 thousand times its weight of air. 



It is remarkable that at pressures up to 

 a few millionths, all the curves are nearly 

 straight lines. This is especially noticeable 

 in the small bulb curves ; showing that at 

 these small pressures the heat-transmitting 

 power of a gas varies directly with its 

 amount. Hence it seems reasonably cer- 

 tain that if the very small fraction of a 

 millionth of the gas examined, which re- 

 mained at the end of each experiment, 

 could have been entirely removed, the heat 

 transmitting power of the vacuum would 

 not have been materially diminished. It 

 was customary at the end of the experi- 

 ments with each gas to close the gauge 

 permanently when the pressure had fallen 

 to a tenth of a millionth or so ; and with 



the capacity of the whole apparatus thus 

 reduced, run the pump continuously from 

 one to two hours. Several sets of observa- 

 tions were always made during this ex- 

 treme exhaustion ; and while the change 

 in the rate of cooling of the thermometer 

 was generally appreciable, it was always 

 very small indeed. In my earlier experi- 

 ments I took the greatest care to insure the 

 absence of mercury vapor in the final 

 vacuum. But the presence or absence of 

 mercury vapor made no difference distin- 

 guishable from the errors of observation. 



Of course, the best vacuum producible by 

 a Sprengel pump still contains many thou- 

 sands of millions of gas molecules per cubic 

 centimater. This may be regai-ded as a 

 prodigiously large or exceedingly small 

 quantity of gas, according to oar point of 

 view. While it has no apparent effect on 

 the general heat-transmitting capacity of 

 the vacuum, it does seem to interfere with 

 or modify some faaction of the ether. This 

 is the only explanation of certain phenom- 

 ena that I can offer. I refer to the differ- 

 ent behavior of the vacua with different 

 residual gases, and in different sized bulbs, 

 in the matter of adherence to, or departure 

 from, Xewtoa's simple law of cooling. The 

 curves a, aa, aaa illustrate these differences 

 in the several cases at the extreme end of 

 section C of the principal carves. These 

 differences are tOD larg3 to be attributed to 

 errors of ob3ervatioa. This is one of sev- 

 eral reasons which lead me to suspect that 

 at higher pressures all the gases examined 

 interfere miterially with and retard the 

 transmission of heat by the ether. In other 

 words, I suspect that the dotted ether line 

 of my curve sheets should not be drawn 

 parallel with the base, and have a constant 

 value at all gaseous pressures, as shown, 

 but should have a decreasing value as the 

 gas pressure rises from zero. On this in- 

 teresting phase of my subject I hope to 

 have more to say at a future date. 



