124 ESSAYS BIOGRAPHICAL AND CHEMICAL 



most : a pound of feathers or a pound of lead ? ' The 

 usual answer is, ' They weigh the same.' 



Although this is strictly true (for a pound is a pound, 

 whether of lead or feathers), a little consideration will 

 show that when the feathers are placed on one pan of a 

 pair of scales and the lead on the other, the lead takes up 

 far less room than the feathers; in other words, the 

 feathers displace much air, while the lead displaces little. 

 That is, the air which the feathers displace no longer 

 rests on the pan ; and if it were still there, the feathers 

 would weigh more. Hence a so-called pound of feathers 

 weighs less than it ought to by the weight of the air 

 displaced. 



Now to overcome this difficulty and to avoid the some- 

 what complicated and uncertain calculations necessary to 

 ascertain the true weight of the things weighed, Sir 

 William Crookes devised a balance closed in by a case in 

 which a vacuum could be made. And it was while 

 obtaining this vacuum that he discovered that light 

 apparently (but really heat) appears to repel certain 

 objects more than others. Thus he was led to experi- 

 ment on vacuum-tubes and to perform all the beautiful 

 experiments which have made his name so famous. At 

 the same time he invented the ' radiometer,' a pretty little 

 toy for showing the repelling action of heat. 



Here again we see the advantage of following up small 

 trails; they may widen to great and most important 

 roads. If Sir William had been content to weigh his 

 compounds of thallium in his vacuum-balance, as most 

 others would have done, and had not had the genius to 

 follow this side-track, he would have missed many of his 

 greatest discoveries. 



A further great step was made when the German 

 physicist Lenard found that Crookes's ' rays ' the ' fourth 

 form of matter ' which he supposed to be repelled from 



