298 THE FLOATING-MATTER OF THE AIR. 



nessi Tt is simply that of stellar space; that is to say, 

 blackness resulting from the absence from the track of the 

 beam of all matter competent to scatter its light. When 

 the flame was placed below the beam, the floating matter 

 was destroyed m situ; and the heated air, freed from this 

 matter, rose into the beam, jostled aside the illuminated 

 particles, and substituted for their light the darkness due to 

 its own perfect transparency. Nothing could more forcibly 

 illusti-ate 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 un- 

 illuminable 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, however, upon the unproved 

 assumption that the air has been affected by the flame. 

 Let a condensed beam be sent through a large flask or 

 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 

 downwards and leave it undisturbed for a day or two. 

 Examined afterwards with the luminous beam, no track 

 is visible ; the light passes through the flask as through 

 a vacuum. The floating matter hns abolished itself, 

 being now attached to the interior surface of the flask, 



' See pp. 3 and 4 of this volume. 



