BACTERIA IN AIR, EARTH, AND WATER. 383 



January, on an average of 4 per two gallons (ten litres) ; the 

 numbers gradually rose until August, when there was an 

 average of 105, and then gradually fell, though he records 

 no observations made during November and December. 

 He had previously confirmed the general results of Miquel, 

 Hare, and others, that as we leave the ground the number 

 of micro-organisms in the air rapidly diminishes. On Nor- 

 wich cathedral spire, at a height of about 300 feet he found 

 in ten litres of air only seven micro-organisms on one occasion, 

 and on the tower, at a height of 180 feet, he found nine, 

 whilst at the base of the cathedral (in the close) eighteen 

 were found. In another series of experiments made at St. 

 Paul's cathedral a similar volume of air taken from the golden 

 gallery yielded eleven, that from the stone gallery thirty-four, 

 and that in the churchyard seventy micro-organisms. He gives 

 a num"ber of other most interesting experiments, for which, 

 however, the reader must refer to the original paper. Car- 

 nelley, Haldane, and Petri have been able to show that the 

 number of micro-organisms in any air depends to a very great 

 extent on the moisture or dryness of the atmosphere, for they 

 found that in the air of sewers, which is necessarily very 

 damp, the number of micro-organisms present is extremely 

 small, unless rapid fermentation is going on, or there is 

 splashing from irregularities in the course of the drain, or from 

 falling in of sewage from a height, the bacteria always 

 tending to settle and to adhere to the moist walls of the drain, 

 so that unless a considerable number of the organisms are 

 carried into the air by the escape of bubbles and gas, or by 

 other agencies, the tendency of these organisms to gravitate 

 allows of their removal from the air. Here, however, it 

 appears to be the moisture on the walls that prevents the 

 escape into the air, rather than any moisture in the air itself. 

 Tyndall demonstrated that exactly the same thing occurred 

 in his chamber coated with glycerine ; bacteria, or other solid 

 particles to which these bacteria were adherent, fell to the 

 floor, or were carried on to the walls, where they were held 

 fast by the glycerine, the air in the chamber thus becoming 

 practically sterile, or deprived of its micro-organisms ; there 

 might still be an enormous number of organisms attached 

 to the floor and walls, but the air itself remained absolutely 

 free, and flasks opened in this chamber would remain sterile 

 for a very considerable, or even an indefinite, period. Griffiths, 



