136 FK'AGMENTS OF SCIENCE. 



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 stretched 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 whiteness. In five minutes there was a sensible 

 diminution of the matter, and in ten minutes it was 

 totally consumed. 



Oxygen, hydrogen, nitrogen, carbonic acid, so 

 prepared 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 ; and the wonder is, not that we 

 should suffer occasionally from its presence, but that 



