652 THE POPULAR SCIENCE MONTHLY. 



the neck of the bulb. Here is an air-pump, and here is the end of the 

 T-piece surrounded by a tube of India-rubber, and here is a pinchcock 

 to close that tube of India-rubber. If you open the pinchcock and 

 work the air-pump with which this end is connected, it is completely 

 exhausted. You may allow it to be filled with air ; you may then 

 open the pinchcock : the air will enter through the cotton-wool, and 

 will fill the bulb. In this way you get the bulb filled, not with com- 

 mon air, but with filtered air. This process is carried on three or four 

 times, so as to make sure that the common air has been displaced by 

 the filtered air. We will suppose that I detach the tube from the air- 

 pump, and other precautions taken. At present, you see the bulb is 

 empty. Taking an infusion of hay, I put the end of the T-piece into 

 the infusion to be introduced into the bulb. The bulb is dipped into 

 hot water; the air expands, and it is driven out. Simply introducing 

 our bulb into cold water, the air shrinks, and by atmospheric pressure 

 the liquid is driven into the bulb. Again we drive the air out, and, 

 by a few operations of this kind, we find that we can charge our bulb 

 with a very great degree of accuracy. You can see the liquid in the 

 bulb at the present time. In this way we charge a bulb which has 

 had its common air and floating matter removed with our infusion. 

 When it is charged, it is very carefully removed, and great precautions 

 are taken so as to prevent any indraught of air. For instance, it is al- 

 ways removed from the cold water, so that, when it is lifted up into the 

 air of the laboratory, a slight expansion shall take place, so that the 

 motion of the air shall be from within outward, instead of from with- 

 out inward. In that way we can, by careful manipulation, obtain 

 bulbs devoid of this floating matter. These are the bulbs you now 

 see before you showing this beautifully pellucid infusion. 



Were this a biological investigation, and not a physical one, I 

 should feel myself out of my element in dealing with it. I leave the 

 determination of the species of bacteria to others far more competent 

 than I am. I can see these organisms and wonder at them when I see 

 them through the microscope; but I have no ability or knowledge to 

 classify them and divide them into species, genera, etc. But these 

 are purely physical experiments, and it is only by such severe experi- 

 ments that this question can be freed from the haze and confusion in 

 which it has been hitherto involved. Even the celebrated Prof. Cohn 

 I say it with the greatest regard and respect for him appears to 

 have no adequate notion of the care necessary to be taken in experi- 

 ments of this kind. To lift a tube out of the boiling liquid and allow 

 it to remain quietly in the air, the entry of the air taking place from 

 without inward, and then, after one or two minutes' exposure, to plug 

 it with cotton-wool, and say that no contamination can reach it, is in 

 my opinion a great mistake. He could not, but by the merest acci- 

 d( nt, get an infusion free from contamination by operating in this 

 way. I have here tubes prepared according to this method. Here 



