318 MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
385. The term “vacuum” has till recently been greatly misapplied. Formerly, an 
air-pump, which would diminish the volume of air in the receiver 1000 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 
when the air is rarefied a few hundred thousand times. 
386- It is generally taken for granted that when a number is divided by 
10,000,000 the quotient must be necessarily small' whereas it may happen that 
the original number is so large that its division by 10,000,000 seems to make 
little impression on it. According to Mr. Johnstone Stoney. the number of 
molecules in a cubic centimetre of air at the ordinary pressure is probably some- 
thing like 1000,000000,000000,000000. Now, when this number is multiplied by 
0‘0000004, or in other words divided by 2,500000, there are still 400,000000,000000 
molecules in every cubic centimetre of gas at the highest exhaustion to which I 
carried the experiment illustrated in fig. 34 — a rarefaction which would correspond to 
the density of the atmosphere about seventy-five miles above the earth’s surface, 
assuming that its density decreases in geometrical progression, as its height increases 
in arithmetical progression ( 2 1 note, 74). 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. 
* Phil. Mag., vol. xxxvi. p. 141. 
