DUST AND DISEASE. 29 



dtist-cone more or less distinct was always revealed by the 

 powerfully condensed electric beam. 



The floating motes resembled minute particles of liquid 

 wMch Lad been carried mechanically from the U-tubes into 

 the expei'imental tube. Precautions were therefore taken to 

 prevent any such transfer. They produced little or no miti- 

 gation. I did not imagine, at the time, that the dust of the 

 external air could find such free passage through the caustic 

 potash and sulphuric acid. This, however, was the case; 

 the jnotes really came from without. They also passed with 

 freedom through a variety of sethers and alcohols. In fact, 

 it requires long-continued action on the part of an acid first 

 to vjet the motes and afterwards to destroy them. By care- 

 fully passing the air through the flame of a spirit lamp, 

 or through a platinum tube heated to bright redness, the 

 floating matter was sensibly destroyed. It was therefore 

 combustible, in other words, organic, matter. I tried to 

 intercept it by a large respirator of cotton-woo'. Close pres- 

 sure was necessary to render the wool eflfective. A plug 

 of the wool, rammed pretty tightly into the tube through 

 which the air passed, was finally found competent to hold 

 back the motes. They appeared from time to time after- 

 wards, and gave me much trouble ; but they were invariably 

 traced in the end to some defect in the purifying apparatus 

 — to some crack or flaw in the sealing-wax employed to render 

 the tubes air-tight. Thus through proper care, but not with- 

 out a great deal of searching out of disturbances, the expe- 

 rimental tube, even when filled with air or vapour, contains 

 nothing competent to scatter the light. The space within it 

 has the aspect of an absolute vacuum. 



An experimental tube in this condition I call optically 

 empty. 



The facts here forced upou my attention had a bear- 

 ing too evident to be overlooked. The inahility of air 

 ■which had heen filtered through cotton-wool to generate 

 microscopic life, had been demonstrated by Schroeder 

 and Pastenr : here the cause of its impotence was ren- 



