308 FRAGMENTS OF SCIENCE. 



carefully turned on. The air passes slowly through the cotton-wool, the 

 caustic potash, and the sulphuric acid in succession. Thus purified it 

 enters the flask F and bubbles through the liquid. Charged with vapor 

 it finally passes into the experimental tube, where it is submitted to ex- 

 amination. The electric lamp L placed at the end of the experimental 

 tube furnished the necessary beam. 



Wanting the cotton-wool the floating matter of the air 

 ran the gantlet of this system. The fact thus forced upon 

 my attention had a bearing too obvious to be overlooked. 

 It rendered at once evident to the senses why air filtered 

 through cotton-wool is incompetent to generate animalcular 

 life. The air is rendered by this treatment optically pure ; 

 in other words, freed from all floating matter, germs in- 

 cluded. But the observations also revealed the great 

 liability to error in experiments of this nature. They 

 showed that without an amount of care which was hardly 

 to be expected in all cases, error would be inevitable. It 

 was especially manifest that the chemical method of 

 Schultze might lead to the most erroneous consequences ; 

 that neither acids nor alkalies had the power of rapid 

 destruction which they had been supposed to possess. In 

 short, the employment of the luminous beam rendered 

 evident the cause of success in experiments rigidly con- 

 ducted like those of Pasteur ; while it made equally evident 

 the certainty of failure in experiments less severely and less 

 skilfully carried out. 



Dr. Bennetts Experiments. 



Take, for example, the well-conceived experiments of 

 Dr. Hughes Bennett, described before the Royal Society 

 of Surgeons in Edinburgh, on January 17, 1868. ' Into 

 flasks containing decoctions of liquorice-root, hay, or tea, 

 Dr. Bennett, by an ingenious method, forced air. The air 

 was driven through two U-tubes, the one containing a so- 



1 British Medical Journal, 13, pt. ii. 1868. 



