PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 519 



cieiitly strong, air is compressed into it, and the weight of that 

 additional air is ascertained. A single stroke of the piston was 

 sufficient to increase the weight of the apparatus, the whole being 

 weighed together by two grains, which turned the scale very per- 

 ceptibly. After four strokes, the air was measured by discharg- 

 ing it into a glass receiver filled with water, when it displaced 

 about one pound of the water. The weight of the atmosphere 

 is thus ascertained to be about l-800th part the weight of water, 

 varying as the barometer varies. 



FUEL. 



Mr. R. L. Pell being called upon to open the subject, remarked : 



The most ancient fuel was wood. It makes a cheerful fire from 

 its bright flame, but is expensive from the fact that it requires 

 frequent renewing,. and much room to store it in. 



Wood consists in longitudinal fibers as fine as hair, the inter- 

 stices between which is filled with cellular tissue, varying with 

 different species, and is composed of carbon, hydrogen and oxy- 

 gen, which, when decomposed by burning, are separated from 

 each other, and reunited in other proportions, and these consti- 

 tute the products of combustion. 



When we place wood on the fire, hydrogen unites with a por- 

 tion of oxygen, and forms carbonated hydrogen, which consti- 

 tutes flame. Other gases are generated, as carbonic oxyde and 

 carbonic acid, by the union of carbon and oxygen, but they do 

 not produce heat or light. After all these are driven ofi" by com- 

 bustion, if we stifle the further burning of the wood, charcoal 

 will remain, which consists of woody fiber, being deprived of all 

 the principles except carbon, which preserves the original form 

 of the wood. If this is burned, its carbon will unite with the 

 oxygen of the atmosphere, and produce carbonic acid. The 

 ashes that remain contain earths, metallic substances and alka- 

 lies. When wood is burned in a chimney, and combustion is 

 nearly at an end, the draft diminishes, and there is then dan- 

 ger that carbonic acid and carbonic oxyde may come out into the 

 apartment and vitiate the air. During combustion there is no 

 sulphuretted hydrogen given off as from coal, because there is no 

 sulphur in it ; but pyroligneous acid is formed, which gives that 

 penetrating effect to smoke. 



The woods that burn the longest and have the densest char- 

 coal are, birch, hazel, hickory, beech, oak and elm. All woods 



