268 Dr. P. F. Frankland and Mr. T. G. Hart. 



in a given volume of air by means of Hesse's process, we have also, as 

 before, determined in each case the number of microbes falling on a 

 given horizontal area in unit of time, by the exposure of small dishes 

 containing sterile nutrient gelatine, as previously described. 



During the hot weather we have experienced considerable difficulty 

 in working with Hesse's tubes, which are veiy liable to melfc in 

 transport and when exposed to the sun ; to obviate this we have made 

 a practice on hot days of surrounding the outer surface of the tube 

 with a coating of bibulous paper saturated with water, and this 

 envelope was again covered with a coating of white tissue paper to 

 prevent the former drying too rapidly. Owing to this precaution 

 we have scarcely ever lost a tube even on the warmest days. 



The greater number of our observations have been made on the 

 roof of the Science Schools, South Kensington, which thus form a 

 continuation of those already recorded. We have also made further 

 experiments in Hyde Park, the Brompton Hospital for Consumption, 

 the Natural History Museum, and in the garden of the latter 

 closely adjoining the traffic in the Exhibition Road, South Ken- 

 sington. 



I. Roof of Science Schools, South Kensington. 



The samples of air were here collected at a height of about 60 feet 

 above the ground in the manner previously described. The 

 particulars of the experiments are recorded in the following table 

 (Table I). 



These experiments, taken together with those already published, 

 form a continuous series, excepting a few breaks, from January to the 

 end of October of the present year, and serve to illustrate the changes 

 which take place in the prevalence of aerial micro-organisms accord- 

 ing to the seasons. 



In order to render these results more readily intelligible, we have 

 also expressed them by means of a curve in the accompanying 

 diagram (Plate 3), in which the ordinates represent the number of 

 micro-organisms found in 10 litres of air, whilst on the horizontal axis 

 the dates are marked off. Below, on the same diagram, the tempera- 

 ture of the air at the time of experiment is recorded. 



From this diagram it will be seen that, although the number of 

 micro-organisms in the air frequently undergoes great changes from 

 one day to another, yet the general tendency is for the number to 

 follow the temperature. Thus, on taking the average of the results 

 obtained in each month, the following sequence is arrived at : — 



