16 THE FLOATING-MATTER OF THE AIR. 



I subsequently found that, to render the air thi's 

 optically pm'e, it was only necessary to leave it to itself 

 for a sufficient time in a small closed chamber, or in a 

 suitably closed vessel. The floating matter gradually 

 attached itself to the top and sides, or sank to the 

 lx>ttom, leaving behind it air possessing no scattering 

 power. Sent through such air, the most concentrated 

 beam failed to render its track visible. 



1 mention ' top ' and ' sides,' as well as ' bottom,' 

 because gravity is not the only agent, perhaps not even 

 the principal agent, concerned in the removal of the 

 floating matter. It is practically impossible to sur- 

 round a closed vessel by an absolutely uniform tempera- 

 ture ; and where differences of temperature, however 

 small, exist, air-currents will be established. By such 

 gentle currents the floating particles are gradually 

 brought into contact with all the surrounding surfaces. 

 To these they adhere, and, no new supply being ad- 

 mitted, the suspended matter finally disappears from 

 the air altogether. 



The striking parallelism of these results with those 

 obtained in the excellent researches of Schwann,* 

 Schroeder and Dusch,^ Schroeder himself,^ and Pasteur* 

 in regard to the question of ' spontaneous generation,' 

 caused me to conclude that the power of scattering 

 light, and the power of producing life, by atmospheric 

 air would be found to go hand in hand. 



This conclusion was strengthened by an experiment 

 easily made and of high significance in relation to this 

 question. It had been pointed out by Professor Lister * 



« Pogg. Ann. 1837, vol. xli. p. 184. 



* Ann. der Pharmacie, vol. Ixxxix. p. 232. 



* Ibid. vol. cix. p. 3.5. 



* Ann. de Chim. et de Phys. 3rd series, vol. Ixiv, p. 83. 



* Introductory Lecture before the University of Edinburgh. 



