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Os the Air-Pump.

ceiver Is füll of that air, lift up the cover, and let down a moufe orbird into the receiver, and the burnt air will immediately kill it. If arändle be let down into the air, it will go out directly ; but, by lettingit down gently, it will purify the air so far as it goes; and so, by let-ting it down more and more, all the air in the receiver will bepurified.

30. Set a bell upon a cushion on the pump-plate, and cover it witha receiver; then shake the pump to make the clapper strike againstthe bell, and the sound will be very well heard : but, exhaust the re-ceiver of air, and then, if the clapper be made to strike ever so hardagainst the bell, it will make no sound at all ; which thews, that airis absolutely necessary for the propagation of sound.

31. Let a candle be placed on one side of a receiver, and viewedthrough the receiver at some distance; then, as soon as the air beginsto be exhausted, the receiver will be stlled with vapours which risefrom the wet leather, by the spring of the air in it; and the light ofthe candle being refracted through that medium of vapours, will havethe appearance of circles of various colours, of a faint relemblance tothofe in the rain-bow.

The air-pump was invented by Otho Guerick of Magdeburg , butwas much improved by Mr. Boyle, to whom we are indebted for oufgreatest part of the knowledge of the wonderful properstes of the air,demonstrated in the above experiments.

The elastic air which is contained in many bodies, and is kept inthem by the weight of the atmosphere, may be got out of them eitherby boiling, or by the air-pump, as shewn in the 2§th experiment: butthe fixed air, which is by much the greater quantity, cannot be gotout but by distillation, fermentation, or putrefaction.

If fixed air did not come out of bodies with difficulty, and spendsome time in extricating itself from them, it would tear them to pieces.'Irees would be -rent by the change of air from a fixt, to an elasticstate, and animals would be burst in pieces by the explosion of air intheir food.

Dr. Hales sound by experiment, that the air in apples is fo much. *Condensed, that if it were let out into the common air, it would fili aspace 48 times as great as the bulk of the apples themfelves j so that itspressure outwards was equal to 11776 lb. and, in a cubic inch of oak,to 19860 lb. against its sides. So that if the air was let loofe at once inthese substances, they would tear every thing to pieces about them witha force superior to that of gunpowder. Hence, in eating apples, it iswell that they part with the air by degrees, as they are chewed, and

R 2 ferment