*4 30 



much ; as long as this enthusiasm can overmatch its 

 mistakes by unequivocal examples of success, so long 

 am I disposed to give it a fair field to work in, and to 

 wish it God speed. 



But let us return to our dust. It is needless to re- 

 mark that it cannot be blown away by an ordinary bel- 

 lows ; or, more correctly, the place of the particles 

 blown away is in this case supplied by others ejected 

 from the bellows, so that the track of the beam remains 

 unimpaired. But if the nozzle of a good bellows be 

 filled with cotton wool not too tightly packed, the air 

 urged through the wool is filtered of its floating matter, 

 and it then forms a clean band of darkness in the illu- 

 minated dust. This was the filter used by Schroeder in 

 his experiments on spontaneous generation, and turned 

 subsequently to account in the excellent researches of 

 Pasteur. Since 1868 I have constantly employed it 

 myself. 



But by far the most interesting and important illus- 

 tration of this filtering process is furnished by the hu- 

 man breath. I fill my lungs with ordinary air and 

 breathe through a glass tube across the electric beam. 

 The condensation of the aqueous vapor of the breath is 

 shown by the formation of a luminous white cloud of 

 delicate texture. It is necessary to abolish this cloud, 

 and this may be done by drying the breath previous to 

 its entering into the beam ; or still more simply, by 

 warming the glass tube. When this is done the lumi- 

 nous track of the beam is for a time uninterrupted. The 

 breath impresses upon the floating matter a transverse mo- 

 tion, but the dust from the lungs makes good the particles 

 displaced. But after some time an obscure disc appears 

 upon the beam, the darkness of which increases, until 



