16 THE POPULAR SCIENCE MONTHLY. 



would bear, and then placed the instrument in a cold room, trusting to 

 the great radiating power of lampblack to maintain the temperature of 

 the blackened surfaces of the vanes below that of the glass. Immedi- 

 ately the wheel began to turn in the opposite direction, and continued 

 to turn until the temperature of the glass came into equilibrium with 

 the surrounding objects. 



These early experiments have since been confirmed to the fullest 

 extent, and no physicist at the present day can reasonably doubt that 

 the radiometer is a very beautiful example of a heat-engine, and it is 

 the first that has been made to work continuously by the heat of the 

 sunbeam. But it is one thing to show that the instrument is a heat- 

 engine, and quite another thing to explain in detail the manner in 

 which it acts. In regard to the last point, there is still room for much 

 difference of opinion, although physicists are generally agreed in refer- 

 ring the action to the residual gas that is left in the bulb. As for my- 

 self, I became strongly persuaded after experimenting with more than 

 one hundred of these instruments, made under my own eye, with every 

 variation of conditions I could suggest that the effect was due to the 

 same cause which determines gas-pressure, and, according to the dynami- 

 cal theory of gases, this amounts to saying that the effect is due to 

 molecular motion. I have not time, however, to describe either my 

 own experiments on which this opinion was first based, or the far more 

 thorough investigations since made by others, which have served to 

 strengthen the first impression. 1 But, after our previous discussions, a 

 few words will suffice to show how the molecular theory explains the 

 new phenomena. 



Although the air in the bulb has been so nearly exhausted that less 

 than the one-thousandth part remains, yet it must be borne in mind 

 that the number of molecules left behind is by no means inconsiderable. 

 As will be seen by referring to our table, there must still be no less 

 than 311,000 million million in every cubic inch. Moreover, the abso- 

 lute pressure which this residual gas exerts is a very appreciable quan- 

 tity. It is simply the one-thousandth of the normal pressure of the 

 atmosphere, that is, of 14 T 7 y pounds on a square inch ; which is equivalent 

 to a little over 100 grains on the same area. Now, the area of the black- 

 ened surfaces of the vanes of an ordinary radiometer measures just 

 about a square inch, and the wheel is mounted so delicately that a 

 constant pressure of one-tenth of a grain would be sufficient to produce 

 rapid motion. So that a difference of pressure on the opposite faces of 

 the vanes, equal to one one-thousandth of the whole amount, is all that 

 we need account for ; and, as can easily be calculated, a difference of 

 temperature of less than half a degree Fahrenheit would cause all this 

 difference in the pressure of the rarefied air. 



But you may ask, How can such a difference of pressure exist on 



1 See notice of these investigations by the author of this article, in American Journal 

 of Science and Arts, September, 18T7 (3), xiv., 231. 



