28 THE FLOATING-MATTER OF THE AIR. 



produce all manner of delusions. Thus my censors, for 

 the most part, have levelled their remarks against posi- 

 tions which were never assumed, and against claims 

 which were never made. The simple history of the 

 matter is this : During the autumn of 1868 I was much 

 occupied with the observations referred to at the be- 

 ginning of this discourse, and in part described in the 

 preceding article. For fifteen years it had been my 

 habit to make use of floating dust to reveal the paths 

 of luminous beams through the air; but until 1868 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, 

 the observations which induced me to give more special 

 attention to the question of spontaneous generation, 

 and the germ theory of epidemic disease, are .thus 

 described : 



The Floating Matter of the Air. 



Prior to the discovery of the foregoing action (the 

 chemical action of light upon vapours), and also during 

 the experiments just referred to, the nature of my work 

 compelled me to aim at obtaining experimental tubes abso- 

 lutely clean upon the interior surface, and absolutely free 

 within from suspended matter. Neither condition is, how- 

 ever, easily attained. 



For however well the tubes might be washed and polished, 

 and however bright and pure they might appear in ordi- 

 nary 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 processes, not con- 

 ducted 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 two U-tubes 

 containing respectively caustic potash and sulphuric acid, a- 



