ON DUST AND DISEASE. 133 



permitted the air to pass through it, ensured the 

 practical contact of the dust with the incandescent 

 metal. The air of the laboratory was permitted to 

 enter the experimental tube, sometimes through the 

 cold, and sometimes through the heated, tube of plati- 

 num. In the first column of the following fragment of 

 a long table the quantity of air operated on is expressed 

 by the depression of the mercury gauge of the air- 

 pump. In the second column the condition of the 

 platinum tube is mentioned, and in the third the state 

 of the air in the experimental tube. 



Quantity of air State of platinum tube State of experimental tube 



15 inches . . Cold . . Full of particles. 

 30 . . Ked-hot . . Optically empty. 



The phrase ' optically empty ' shows that when the 

 conditions of perfect combustion were present, the 

 floating matter totally disappeared. 



In a cylindrical beam, which strongly illuminated 

 the dust of the laboratory, I placed an ignited spirit- 

 lamp. Mingling with the flame, and round its rim, 

 were seen curious wreaths of darkness resembling an 

 intensely black smoke. On placing the flame at some 

 distance below the beam, the same dark masses stormed 

 upwards. They were blacker than the blackest smo^e 

 ever seen issuing from the funnel of a steamer ; and 

 their resemblance to smoke was so perfect as to lead 

 the most practised observer to conclude that the 

 apparently pure flame of the alcohol lamp required but 

 a beam of sufficient intensity to reveal its clouds of 

 liberated carbon. 



But is the blackness smoke ? This question pre- 

 sented itself in a moment and was thus answered : A 

 red-hot poker was placed underneath the beam : from 



