132 FRAGMENTS OF SCIENCE. 



sulphuric acid which, while yielding no vapour of its own, 

 powerfully absorbs the aqueous vapour of the air. 1 To 

 my astonishment, the air of the Eoyal Institution, sent 

 through these tubes at a rate sufficiently slow to dry 

 it, and to remove its carbonic acid, carried into the 

 experimental tube a considerable amount of mechani- 

 cally suspended matter, which was illuminated when 

 the beam passed through the tube. The effect was 

 substantially the same when the air was permitted to 

 bubble through the liquid acid, and through the solu- 

 tion of potash. 



I tried to intercept this floating matter in various 

 ways; and on October 5, 1868, prior to sending the 

 air through the drying apparatus, it was carefully 

 permitted to pass over the tip of a spirit-lamp flame. 

 The floating matter no longer appeared, having been 

 burnt up by the flame. It was therefore organic 

 matter. I was by no means prepared for this result ; 

 having previously thought that the dust of our air was, 

 in great part, inorganic and non-combustible. 2 



I had constructed a small gas-furnace, now much 

 employed by chemists, containing a platinum tube, 

 which could be heated to vivid redness. 3 The tube 

 contained a roll of platinum gauze, which, while it 



1 The apparatus is figured at p. 98. 



2 According to an analysis kindly furnished to me by Dr. Percy, 

 the dust collected from the walls of the British Museum contains 

 fully 50 per cent, of inorganic matter. I have every confidence in 

 the results of this distinguished chemist ; they show that the float- 

 ing dust of our rooms is, as it were, winnowed from the heavier 

 matter. As bearing directly upon this point I may quote the fol- 

 lowing passage from Pasteur : ' Mais ici se presente une remarque : 

 la poussiere que 1'on trouve a la surface de tous les corps est soumise 

 constamment & des courants d'air, qui doivent soulever des parti- 

 cules les plus legeres, au nombre desquelles se trouvent, sans doute, 

 de preference les corpuscules organises, oeuf s ou spores, moins lourds 

 generalement que les particules minerales.' 



8 Pasteur was, I believe, the first to employ such a tube. 



