DUST AND DISEASE. 305 



varied and ingenious attack. On this point I will only say- 

 that when feeling escapes from behind the intellect, where 

 it is a useful urging force, and places itself in front of the 

 intellect, it is liable to produce glamour and all manner 

 of delusions. Thus my censors, for the most part, have lev- 

 elled their remarks against positions which I never assumed, 

 and against claims which I never made. The simple his- 

 tory of the matter is this : During the autumn of 1868 I 

 was much occupied with the observations referred to at the 

 beginning of this discourse. For fifteen years I had ha- 

 bitually employed the electric light, making use of the 

 floating dust to reveal the paths of luminous beams ; but 

 until 1868, when I was driven to it, I did not intentionally 

 reverse the process and employ a luminous beam to reveal 

 and examine the dust. In a paper presented to the Royal 

 Society in December, 1869, I thus described the observa- 

 tions which induced me to give more special attention to 

 the question of spontaneous generation and the germ- 

 theory of epidemic disease. 



The Floating Matter of the Air. 



Prior to the discovery of the foregoing action (the chemical action of 

 light upon vapors), and also during the experiments just referred to, the 

 nature of my work compelled me to aim at obtaining experimental tubes 

 absolutely clean upon the surface, and absolutely empty within. Neither 

 condition is, however, easily attained. 



For however well the tubes might be washed and polished, and how- 

 ever bright and pure they might appear in ordinary daylight, the electric 

 beam infallibly revealed signs and tokens of dirt. The air was always 

 present, and it was sure to deposit some impurity. All chemical pro- 

 cesses, not conducted in a vacuum, are open to this disturbance. When 

 the experimental tube was exhausted it exhibited no trace of floating 

 matter, but on admitting the air through the TJ-tubes containing caustic 

 potash and sulphuric acid, a dust-cone more or less distinct was always re- 

 vealed by the powerfully-condensed electric beam. 



The floating motes resembled minute particles of liquid which had 



