

ON DUST AND DISEASE. 169 



is taken to defend the eye from the access of all other 

 light, and, thus defended, it becomes an organ of in- 

 conceivable delicacy. Indeed, an amount of impurity 

 so infinitesimal as to be scarcely expressible in numbers, 

 and the individual particles of which are so small as 

 wholly to elude the microscope, may, when examined 

 by the method alluded to, produce not only sensible, 

 but striking, effects upon the eye. 



We will apply the method, in the first place, to an 

 experiment of M. Pouchet intended to prove conclu- 

 sively that animalcular life is developed in cases where 

 no antecedent germs could possibly exist. He produced 

 water from the combustion of hydrogen in air, justly 

 arguing that no germ could survive the heat of a 

 hydrogen flame. But he overlooked the fact that his 

 aqueous vapour was condensed in the air, and was 

 allowed as water to trickle through the air. Indeed the 

 experiment is one of a number by which workers like 

 M. Pouchet are differentiated from workers like Pasteur. 

 I will show you some water, produced by allowing a 

 hydrogen flame to play upon a polished silver condenser, 

 formed by the bottom of a silver basin, containing ice. 

 The collected liquid is pellucid in the common light ; 

 but in the condensed electric beam it is seen to be laden 

 with particles, so thick- strewn and minute as to produce 

 a continuous luminous cone. In passing through the 

 air the water loaded itself with this matter ; and the 

 deportment of such water could obviously have no in- 

 fluence in deciding this great question. 



We are invaded with dirt not only in the air we 

 breathe, but in the water we drink. To prove this I 

 take the bottle of water intended to quench your 

 lecturer's thirst; which, in the track of the beam, 

 simply reveals itself as dirty water. And this water is 

 no worse than the other London waters. Thanks to the 



