828 FRAGMENTS OF SCIENCE 



171 situ; and the heated air, freed from this matter, rose into 

 the beam, jostled aside the illuminated particles, and substi- 

 tuted for their light the darkness due to its own perfect 

 transparency. Nothing could more forcibly illustrate the 

 invisibility of the agent which renders all things visible. 

 The beam crossed, unseen, the black chasm formed by the 

 transparent air, while, at both sides of the gap, the thick- 

 strewn particles shone out like a luminous solid under the 

 powerful illumination. * 



Supposing an infusion intrinsically barren, but readily 

 susceptible of putrefaction when exposed to common air, 

 to be brought into contact with this unilluminable air, 

 what would be the result? It would never putrefy. It 

 might, however, be urged that the air is spoiled by its 

 violent calcination. Oxygen passed through a spirit-lamp 

 flame is, it may be thought, no longer the oxygen suitable 

 for the development and maintenance of life. "We have 

 an easy escape from this difficulty, which is based, how- 

 ever, upon the unproved assumption that the air has been 

 affected by the flame. Let a condensed beam be sent 

 through a large flask of bolthead containing common air. 

 The track of the beam is seen within the flask — the dust 

 revealing the light, and the light revealing the dust. Cork 

 the flask, stuff its neck with cotton- wool, or simply turn 

 it mouth downward and leave it undisturbed for a day or 

 two. Examined afterward with the luminous beam, no 

 track is visible; the light passes through the flask as 

 through a vacuum. The floating matter has abolished 

 itself, being now attached to the interior surface of the 

 flask. Were it our object, as it will be subsequently, to 

 effectually detain the dirt, we might coat that surface with 



^ See page 142, voL I 



