PROCEEDINOS OP THE POLYTECHNIC ASSOCIATION. 503 



complete the circuit. If now one conducting wire be slowlj 

 raised, a small chain will be formed by the juxtaposition of par- 

 ticles will be drawn out, and if care is taken, every particle will 

 he drawn out in one long chain. Cauderay does not believe the 

 adhesion due to magnetism but to a superficial fusion of the filings. 

 He has shown that these particles offer very great resistance to 

 dynamic electricity, and upon this principle he has constructed a 

 very powerful rheostat- 



Ventilation. 



The chairman opened the discussion of this subject by some 

 remarks on the composition of common air. 



In one hundred parts, by measure, are oxygen, 20.61 parts; 

 nitrogen, 77.95 parts- carbonic acid gas, or carbonic anhydride, 

 .04 of one part Aqueous vapor variable with the temperature, 

 the mean being about 1.40 parts. At 59° F. the quantity of vapor 

 required to saturate in a given volume of air, is twice as much as 

 at 32° F; at 86'^ F. it is three times that required at 32°. Traces 

 of ammonia and carburetted hydrogen or marsh gas are found in 

 the air; and that enveloping manufacturing towns often is slightly 

 contaminated with sulphuretted hydrogen and sulphuric acid. 

 Traces of nitric acid are also detected, which are supposed to 

 result from lightning, chemically combining nitrogen and oxygen. 



The proportions of nitrogen and oxygen in the air do not mate- 

 rially vary in hot or cold, moist or dry climates. The proportion 

 of carbonic acid is preserved by the wants of the vegetable and 

 animal world. The animal is sustained by inspiring air and 

 abstracting from it oxygen to unite with carbon and form carbonic 

 acid gas; while the plant, under the influence of light, absorbs 

 carbonic acid and give off pure oxygen. Thus by the respiration 

 of leaf and lung, the atmospheric equilibrium is maintained. 



The proportion of carbonic acid in the air, averaging only about 

 four parts in ten thousand, seems very small; yet it has been esti- 

 mated that the whole amount of carbon in the atmosphere is 

 greater than all found in the vegetable and animal kingdoms, and 

 probably in all the carbonates forming part of the earth's crust. 

 Inspired air contains four hundredths of one per cent, of carbonic 

 acid, and expired air from three and one-third to three and a half 

 per cent., that is, the quantity of carbonic acid exhaled is about 

 eighty-five times more than that inhaled. The amount contained 

 in the air of a room occupied by one or more persons, can never 



