270 Dr. P. F. Frankland and Mr. T. G. Hart, 



made in the open air at other places than the roof of the Science 

 Schools. This table includes, in the first place, a comparison between 

 the number of micro-organisms present in the air of Hyde Park, the 

 roof of the Science Schools, and the entrance to the latter in the 

 Exhibition Road respectively, the experiments in these three places 

 being all made on the same day. It will be seen that the air in Hyde 

 Park contained markedly less than either of the other two, and that 

 the air in the Exhibition Road in which a large amount of traffic was 

 going on at the time was considerably richer in micro-organisms than 

 the air on the roof. 



Tien follow several experiments made in the garden of the 

 Natural History Museum in the immediate vicinity of the Exhibition 

 Road. In every case, excepting one, this air was exceedingly rich in 

 micro-organisms, as was to be anticipated ; whilst on the occasion 

 when the number of organisms was small, the wind, which was very 

 gentle, was not blowing from the road but over the grass of the 

 garden which was damp at the time. 



The above experiments are intended to form a supplement to those 

 already published by one of us, in which the same method of investiga- 

 tion was pursued. In the course of these experiments we have found 

 that the results obtained with Hesse's apparatus are liable to 

 considerable inaccuracy when the latter is employed in a disturbed 

 atmosphere, more especially when the aerial . currents are irregular in 

 direction. This source of error has been fully discussed in another 

 paper by one of us, and a new method of examining air for micro- 

 organisms, in which this difficulty is overcome, has been devised and 

 its accuracy carefully tested. 



[Note. — Since the communication of the above, we have completed 

 the observations for November and December, with the following 

 results : — 



Average number 

 of colonies 



Average temperature obtained from 10 litres 

 at time of of air by 



1886. experiments. Hesse's method. 



November 9'4° 13 



December . . 4*4 20 



March 5, 1887.] 



