DUST AND DISEASE. 279 



tered. In the second column the condition of the platinum 

 tube is mentioned, and in the third the state of the air 

 which had entered the experimental tube. 



Quantity of Air. State of Platinum Tube. State of Experimental Tube. 



15 inches Cold Full of particles. 



15 " Red hot Optically empty. 



The phrase " optically empty " shows that when the con- 

 ditions of perfect combustion were present, the floating 

 matter totally disappeared. It was wholly burnt up, leav- 

 ing no sensible residue. The experiment was repeated 

 many times, with the same invariable result. 



The whole of the visible particles floating in the air of 

 London rooms being thus proved to be of organic origin, 1 

 I sought to burn them up at the focus of a concave re- 

 flector. One of the powerfully convergent mirrors em- 

 ployed in my experiments on combustion by dark rays was 

 here made use of, but I failed in the attempt. Doubtless 

 the floating particles are in part transparent to radiant heat, 

 and are so far incombustible by such heat. Their rapid 

 motion through the focus also aids their escape. They do 

 not linger there sufficiently long to be consumed. A flame 

 it was evident would burn them up, but I at first thought 

 the presence of the flame would mask its own action among 

 the particles. 



1 According to an analysis kindly furnished to me by Dr. Percy, the dust 

 collected from the walk of the British Museum contains fully 50 per cent, 

 of inorganic matter. I have every confidence in the results of this dis- 

 tinguished chemist ; they show that the floating dust of our rooms is, as it 

 were, winnowed from the heavier matter. As bearing directly upon this 

 point, I may quote the following 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 a des courants d'air, qui doivent sou- 

 lever ses particules les plus legeres, au nombre desquelles se trouvent, sans 

 doute, de preference les corpuscules organises, oaufs ou spores, moins 

 lourds generalement que les particules minerales." 



