126 Dr. J. M. Hobson on 



Frankland, who found that certain forms could not only live but 

 multiply in distilled water. Very pure deep well-water also, which 

 may contain but five organisms to the cubic centimetre, after 

 keeping in a closed sterilized vessel for a few days, may contain 

 as many as 500,000 in the same space. The few bacteria which 

 have gained an access to this virgin soil can thus multiply with- 

 out check. Every pinch of garden mould contains enormous 

 numbers of bacteria, though some of these may be amongst 

 those which can thrive in water. It cannot be supposed that any 

 bacteria are really aerial in their habitat, for they would lack the 

 moisture they require for growth for one thing ; yet, except at very 

 great heights, or well out to sea, it is not possible to examine any 

 considerable volume of air without finding any. The reason for 

 this clearly is that bacteria are so exceedingly minute, and are so 

 intimately associated with the multitudinous things that give off 

 dust, that they are carried upwards by various currents and mingled 

 with the mass of the atmosphere by the winds. It is well known 

 that some bacterial cells will survive a long period of drying, 

 that the spores are much more proof against this form of 

 destruction, and that either, directly they light upon a spot where 

 the requisite warmth, moisture, and nourishment are combined, 

 will vegetate and multiply. It is therefore not surprising that 

 under these circumstances bacteria should find their way to 

 wherever non-living organic solids or liquids exist. Whether 

 they should multiply in these situations depends upon circum- 

 stances. It does not appear, however, that they can be found 

 within the blood or solid tissues of perfectly normal living beings. 

 Their presence in the mouth, intestines, &c., of living animals is 

 not an exception to the rule, for these parts are, strictly speaking, 

 outside the living tissues. 



Effects of Oxygen and Light. — Pasteur used the terms 

 aerobic and an-aerobic to denote the extremes in relation to 

 oxygen, the former indicating organisms to which the presence 

 of plenty of oxygen was essential to their growth, while the 

 latter implied that the organisms could get their oxygen out of 

 the organic compounds in their nutrient medium, and that the 

 presence of free oxygen reduced their vegetation to a minimum 

 or stopped it. Typical of the aerobic forms is the hay bacillus, 

 B. subtilis ; of the anaerobic, the various putrefactive organisms 

 and the bacillus of tetanus. There are intermediate conditions, 

 however, and when I come to speak of cholera I shall show how 

 the characteristics of the organism can be modified by aerobic 

 and anaerobic growth respectively. Oxygen has also a powerful 

 effect upon the movements of bacteria, attracting the aerobics 

 and repelling the anaerobics. De Bary quotes a very pretty ex- 

 periment illustrating this. A piece of alga and bacteria are placed 

 in water containing no free oxygen. As the alga begins liberating 



