THE BRITISH MEDICAL ASSOCIATION 175 



development of such organisms as the microscope enables us to detect, and 

 the concomitant putrefactive changes, are occasioned by particles of dust 

 suspended in the atmosphere, but not by the atmospheric gases. I confess, Mr. 

 President, I am ready to blush for the character of our profession for scientific 

 accuracy when I see the loose comments sometimes made upon this experiment ; 

 and I am tempted to doubt whether some of the commentators can have enjoyed 

 the advantages of sufficient education either in chemical physics or in logic. 

 The simplicity and perfect conclusiveness of the experiment constitute its 

 great charm, and render it, as it appears to me, deserving of your careful con- 

 sideration. Yet, having before published an account of it, although nearly 

 two years have since elapsed, so as to add considerably to its weight, I do not 

 know that I should have felt justified in bringing it forward on the present 

 occasion, if I had not an additional fact to communicate respecting it besides 

 the results of further lapse of time. We have seen that we have been forced 

 to the conclusion that, though the gases of the air certainly pass into the body 

 of the flask and out again every twenty-four hours, its dust, even though of 

 extreme minuteness, must be arrested by the contorted tube. Now, inevitable 

 as this inference is, it will be satisfactory to have it converted into the position 

 of an observed fact. This Professor Tyndall's simple but beautiful mode of 

 investigation with a condensed beam of light has lately enabled me to do. 

 Having prepared two dry glass flasks, one of them having the neck drawn out 

 and contorted, I arranged them, through the kind assistance of my colleague, 

 Professor Tait, so that the body of each was pierced by a beam of highh' con- 

 densed sunlight in an otherwise dark apartment. The beam, scattered by the 

 floating particles of dust, showed white in the surrounding darkness, within 

 the flasks as well as without, proving that the air within the flasks was dusty 

 like that outside. I now closed with sealing-wax the orifice of the unbent 

 flask, and, leaving the other open, allowed both to remain undisturbed in the 

 laboratory. A fortnight later I again submitted them to the solar beam, 

 condensed as before, and now found that in both flasks alike the visible part 

 of the beam terminated abruptly at the glass on each side, showing that in 

 both the air was, as Tyndall expresses it, ' opticallv empty.' or, in other words, 

 that it was destitute of even such minute particles of floating matter as could 

 produce the faintest nebulosity. During the time between the two obser\ations, 

 the force of gravity had led to the subsidence of even the minutest floating 

 particles ; and, though the changing temperature of the kiboratory had of 

 necessit}^ induced the daily entrance of air into the open flask, the bent form 

 and fine calibre of the tube by which it was admitted had effectually filtered it 

 of suspended material, though in a very dusty apartment. 



