300 THE FLOATING -MATTER OF TILE AIR 



constructed, having glass fronts, side-windows, and 

 back-doors. Through the bottoms of the chambers 

 test-tubes pass air-tight ; their open ends, for about 

 one-fifth of the length of the tubes, being within the 

 chambers. Provision is made for a free connexion 

 through sinuous channels between the inner and the 

 outer air. Through such channels, though open, no 

 dust will reach the chamber. The top of each chamber 

 is perforated by a circular hole two inches in diameter, 

 closed air-tight by a sheet of india-rubber. This is 

 pierced in the middle by a pin, and through the pin- 

 hole is pushed the shank of a long pipette, ending 

 above in a small funnel. The shank also passes through 

 a stuffing-box of cotton-wool moistened with glycerine ; 

 so that, tightly clasped by the rubber and wool, the 

 pipette is not likely in its motions up and down to carry 

 any dust into the chamber. The annexed woodcut (fig. 24) 

 shows a chamber, with six test-tubes, its side-windows 

 IV w, its pipette p c, and its sinuous channels a b which 

 connect the air of the chamber with the outer air. 



The chamber is carefully closed and permitted to 

 remain quiet for two or three days. Examined at the 

 beginning by a beam sent through its windows, the air 

 is found laden with floating matter, which in three 

 days has wholly disappeared. To prevent its ever 

 rising again, the internal surface of the chamber was 

 at the outset coated with glycerine. The fresh but 

 putrescible liquid is introduced into the six tubes in 

 succession by means of the pipette. Permitted to 

 remain without furtlier precaution, every one of the 

 tubes would putrefy and fill itself with life. The liquid 

 has been in contact with the dust-laden air outside by 

 which it has been infected, and the infection must be 

 destroyed. This is done by plunging the six tubes into 

 a bath of heated oil and boiling the infusion. The time 



