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II. The Optical Deportment of the Atmosphere in relation to the Phenomena 
of Putrefaction and Infection. By John Tyndall, F.JR.S. 
Delivered orally January 13, — Received complete April 6, 1876. 
§ 1. Introduction. 
An inquiry into the decomposition of vapours by light, begun in 1868 and continued in 
1869*, in which it was necessary to employ optically pure air, led me to experiment 
on the floating matter of the atmosphere. A brief section of a paper published in the 
Philosophical Transactions for 1870 f is devoted to this subject. 
I at that time found that London air, which is always thick with motes, and also 
with matter too fine to be described as motes, after it had been filtered by passing it 
through densely packed cotton-wool, or calcined by passing it through a red-hot 
platinum-tube containing a bundle of red-hot platinum wires, or by carefully leading it 
over the top of a spirit-lamp flame, showed, when examined by a concentrated lumi- 
nous beam, no trace of mechanically suspended matter. The particular portion of 
space occupied by such a beam was not to be distinguished from adjacent space. 
The purely gaseous portion of our atmosphere was thus shown to be incompetent to 
scatter light. 
I subsequently found that, to render the air thus optically pure, it was only neces- 
sary to leave it to itself for a sufficient time in a closed chamber, or in a suitably closed 
vessel. The floating matter gradually attached itself to the top and sides, or sank to the 
bottom, leaving behind it air possessing no scattering power. Sent through such air, 
the most concentrated beam failed to render its track visible. 
I mention 4 top ’ and 4 sides,’ as well as 4 bottom,’ because gravity is not the only agent, 
probably not the principal agent, concerned in the removal of the floating matter. It is 
practically impossible to surround a closed vessel by an absolutely uniform temperature ; 
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 admitted, 
the suspended matter finally disappears from the air altogether. 
The parallelism of these results with those obtained in the excellent researches of 
Schwann J, Scheoeder 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 the air would be found to go hand in hand. 
* Proc. Roy. Soc. vol. xvii. § Ann. der Pharmacie, vol. lxxxix. p. 232. 
+ Yol. clx. p. 337. || Ann. der Pharmacie, vol. cix. p. 35. 
+ Pogg. Ann. 1837, vol. xli. p. 184. Ann. de Chim. et de Phys. 3rd series, vol. lxiv. p. 83. 
F 
MDCCCLXXVI. 
