194 THE SEVEN FOLLIES OF SCIENCE 



phere, which, as a resisting medium, under some condi- 

 tions, is almost 'as effective as so much sand. On a stone 

 slab three feet square there rests a load of air weighing 

 nearly nine tons. Now this air, if moved slowly, does not 

 offer much resistance to the moving agent. The most 

 delicate fan, if moved very slowly in the air, does not even 

 bend. But if moved rapidly it bends very perceptibly, 

 and if moved with great velocity it will be broken. We 

 can easily see, therefore, that when an effort is made to 

 move nine tons of air with the velocity of the gases evolved 

 by exploded dynamite, the air will offer almost the resist- 

 ance of a solid body, and a stone slab, though hard and 

 strong, breaks under the blow. 



THAT THE ART OF HARDENING COPPER 

 IS LOST 



T short intervals there appears in our different 

 periodicals an article telling us that somebody 

 has found a lot of old copper tools hard enough 

 to cut the hardest stone and bewailing the fact 

 that the process by which these tools were hardened by 

 some prehistoric race is now unknown and must be classed 

 amongst the so-called "lost arts." 



That the Egyptians and some other peoples knew how 

 to harden copper is unquestionably true, but a chemical 

 analysis of their tools quickly revealed the secret, and there 

 has never been a time since then when we could not pro- 

 duce copper tools quite as good as those of the ancients, 

 and probably better. During his investigations into metal- 

 lic alloys suitable for cutlery, Faraday produced an alloy 



