226 FERGUSON'S LECTURES. 



LECT. cover it with a receiver ; then shake the pump to make 

 the clapper strike against the bell, and the sound will 

 be very well heard : but, exhaust the receiver of air, and 

 then, if the clapper be made to strike ever so hard 

 against the bell, it will make no sound at all ; which 

 shews, that air is absolutely necessary for the propaga- 

 tion of sound.* 2 



31. Let a candle be placed on one side of a receiver 

 and viewed through the receiver at some distance : then, 

 as soon as the air begins to be exhausted, the receiver will 

 be filled with vapours which rise from the wet leather, 

 by the spring of the air in it ; and the light of the can- 

 dle being refracted through that medium of vapours, will 

 have the appearance of circles of various colours, of a 

 faint resemblance to those in the rainbow. 



The air-pump was invented by Otto Guericke of Mag- 

 deburg, but was much improved by Mr. Boyle, to whom 

 we are indebted for our greatest part of the knowledge 

 of the wonderful properties of the air, demonstrated in 

 the above experiments. 



The elastic air which is contained in many bodies, 

 and is kept in them by the weight of the atmosphere, 

 may be got out of them either by boiling, or by the air- 

 pump, as shewn in the 25th experiment ; but the fixed 

 air which is by much the greater quantity, canrffctbe got 

 out but by distillation, fermentation, or putrefaction. 



If fixed air did not come out of bodies with difficulty, 

 and spend some time in extricating itself from them, it 

 would tear them to pieces. Trees would be rent by the 

 change of air from a fixed to an elastic state, and animals 

 would be burst in pieces by the explosion of air in their 

 food. 



Dr. Hales found, by experiment, that the air in apples 

 is so much condensed, that if it were let out into the 

 common air, it would fill a space of 48 times as great as 



Note 62. The above assertion is a vulgar error r as sound may be 

 transmitted by most bodies. Air, indeed, is the ordinary vehicle by 

 which it is propagated, but it is not the only medium. 



