6 THE FLOATING MATTER OF THE AIK. 



On the wooden base of a cubical glass shade, a 

 cubic foot in volume, upright supports were fixed, and 

 from one support to the other 38 inches of platinum 

 wire were stietclied in four parallel lines. The ends 

 of the platinum wire were soldered to two stout copper 

 wires which passed through the base of the shade 

 and could be connected with a battery. As in the 

 last experiment the shade rested upon cotton-wool. 

 A beam sent through the shade revealed the suspended 

 matter. The platinum wire was then raised to white- 

 ness. In five minutes there was a sensible diminution 

 of the matter, and in ten minutes it was totally con- 

 sumed. 



Oxygen, hydrogen, nitrogen, carbonic acid, so pre- 

 pared as to exclude all floating particles, produce, 

 when poured or blown into the beam, the darkness of 

 stellar space. Coal-gas does the same. An ordinary 

 glass shade, placed in the air with its mouth down- 

 wards, permits the track of the beam to be seen 

 crossing it. When coal-gas or hydrogen is allowed to 

 enter the shade by a tube reaching to its top, the gas 

 gradually fills the shade from above downwards. As 

 soon as it occupies the space crossed by the beam, the 

 luminous track is abolished. Lifting the shade so as 

 to bring the common boundary of gas and air above 

 the beam, the track flashes forth. After the shade is 

 full, if it be inverted, the pure gas passes upwards like 

 a black smoke among the illuminated particles. 



The Germ Theory of Contagious Disease, 



There is no respite to our contact with the floating 

 matter of the air. We not only suffer from its 

 meclianical irritation, but it is a growing belief that 

 a portion of it lies at the root of a class of disorders 



