July 5, 1888] 



NATURE 



233 



made with Hesse's method, which consists in aspirating air 

 through glass tuhes about 3 feet in length, coated internally with 

 a film of gelatine-peptone. The organisms, owing to the pro- 

 perty they possess of rapidly subsiding in the absence of disturb- 

 ing influences, fall on the surface of the gelatine, and give rise to 

 colonies. 



The following series of observations was made by this 

 method in 1886 1 on the roof of the Science Schools, South 

 Kensington Museum, in order to trace the seasonal variations in 

 the number of micro-organisms present in the air of one and the 

 same place. The following are the averages obtained for each 

 month during which these observations were made : — 



Number of Micro-organisms found in Ten Litres ( Two Gallons') 

 of Air. 



From these figures it will be seen that it is during the 

 summer that the largest number of micro-organisms are found 

 in the air, whilst the smallest average number was recorded 

 in the month of January. 



The air at sea, the air at higher altitudes, and the air in 

 sewers, have all been explored by means of Hesse's method. 



Thus Dr. Fischer, 2 in experiments carried on at sea, found 

 that beyond a distance of 120 sea miles from land micro- 

 organisms were invariably absent. And, inasmuch as micro- 

 organisms are abundantly present in sea-water, it thus appears 

 that no micro-organisms are communicated to the air from the 

 water even when the latter is much disturbed. Moreover, as 

 might have been anticipated, this complete freedom from micro- 

 organisms was attained even in close proximity to land, pro- 

 vided the wind had passed over the above-mentioned distance 

 of sea. 



As regards the air at higher altitudes, experiments have been 

 made on the dome of St. Paul's, in London, and on the spire of 

 Norwich Cathedral, which show that even in ascending to such 

 modest elevations in densely-populated centres, the number of 

 micro-organisms suspended in the air undergoes very marked 

 diminution. 



Thus, on the top of Norwich Cathedral spire, at a height of 

 about 300 feet, I found in ten litres (two gallons) of air only seven 

 micro-organisms, and on the tower, about 180 feet high, I 

 found nine, whilst at the base of the. Cathedral, in the Close, 

 eighteen were found. These results are fully confirmed by 

 another series of experiments made at St. Paul's Cathedral. In 

 this case the air examined from the Golden Gallery yielded in the 

 same volume eleven, that from the Stone Gallery thirty-four, 

 whilst in the churchyard there were seventy micro-organisms 

 present. 



The contrast between town and country air, and even between 

 the air of the London parks and streets is also exceedingly 

 sharp. In Hyde Park — the place selected for the experiment 

 being as far removed from roads and traffic as possible — I found 

 eighteen, whilst on the same day, June 7, the air in the 

 Exhibition Road, South Kensington, yielded as many as ninety- 

 four. On the following day, however, when the traffic was very 

 great, and the air was consequently heavily laden with dust, the 

 number rose to 554. This is in marked contrast to the microbial 

 condition of country air, for on the Surrey Downs in the same 

 volume only two micro-organisms were found ; and in the case 

 of an extensive heath near Norwich only seven. 



Within doors we find that the number of micro-organisms 

 suspended in the air depends, as we should have expected, upon 

 the number of people present, and the amount of disturbance of 

 the air which is taking place. Thus, on examining the air in 

 the large entrance hall of the Natural History Museum in 

 Cromwell Road it was found to yield under ordinary conditions 

 from fifty to seventy organisms in the same volume (two gallons), 

 but on Whit Monday, when an immense number of visitors were 

 present in the building, I found as many as 280. Again, on a 

 paying day at the South Kensington Museum, about eighteen 

 micro-organisms were found, but on the Saturday, when no en- 



1 " The Distribution of Micro-organisms in Air," Proc. Roy. Soc, No. 245, 

 1885 ; " Further Experiments on the Distribution of Micro-organisms in Air," 

 Froc. Roy. Soc, vol. xlii. p. 267, 1886. * 



2 " Bacteriologische Untersuchungen auf einer Reise nach Westindien, ' 

 Zeitschrift fur Hygiene, Bd. i. Heft 3. 



trance fee is charged, there were as many as seventy-three in the 

 same volume of air. 



The air of sewers has been shown by Carnelley in this country, 

 and by Petri in Berlin, to be remarkably free from micro- 

 organisms, the number being almost invariably less than iii 

 outside air. That this should be the case is only natural when 

 the moist nature of the walls and the absence of dust in these 

 subterranean channels is borne in mind, and although their 

 liquid contents is teeming with bacterial life, there is no reason 

 why the latter should be carried into the air provided no effer- 

 vescence or splashing takes place. On the other hand, if the 

 contents of a sewer enter into fermentation and bubbles of 

 gas become disengaged, minute particles of liquid with the living 

 matter present may be carried to great distances, and it must 

 not, therefore, be too hastily concluded that because sewer air 

 is generally remarkably free from micro-organisms, that, there- 

 fore, a visit to the sewers should be attended with such beneficial 

 results as atrip to sea or the ascent of a mountain summit ! 



During the use of Hesse's method I became acquainted with, 

 several serious defects which it possessed, and in order to over- 

 come these disadvantages I was led to devise a new process x for 

 the examination of air. This consists essentially in aspirating 

 a given volume of air through a small glass tube, not more than, 

 4 inches long and \ inch in width, which is provided with two 

 filter-plugs, the first of which is more pervious than the second, 

 and consists of glass-wool coated with sugar, whilst the second 

 contains, in addition, a layer, \ inch in thickness, of fine sugar- 

 powder. On these plugs the microbes suspended in the aspirated 

 air are deposited, and each of these plugs is then introduced 

 into a separate flask containing a small quantity of melted gela- 

 tine-peptone ; with this the plug is agitated until it becomes 

 completely disintegrated, and since the sugar-coating of the glass- 

 wool dissolves in the liquid gelatine, the microbes become 

 immediately detached. The contents of the flask are then made 

 to congeal in the form of a thin film over its inner surface. The 

 flasks are then preserved at a suitable temperature, and in the 

 course of a few days the colonies derived from the organisms, 

 which were collected by the plug, make their appearance and 

 can be counted and further studied. Now, if the plug has been- 

 properly constructed, the flask into which the second or more 

 impervious plug has been introduced will be found to remain 

 quite sterile, clearly showing that the first plug has arrested all 

 the microbes suspended in the aspirated air. This method yields 

 results which agree not only very closely amongst themselves, but 

 also with those obtained by Hesse's method, if the experiments 

 are made in still air, which is the condition necessary for an 

 accurate result being obtained with a Hesse tube. As this new 

 method is equally applicable in disturbed air, it possesses great 

 advantages over Hesse's, and is, moreover, considerably more 

 convenient, as it renders possible the examination of a far larger 

 volume of air in a very much shorter space of time, the apparatus 

 required being also exceedingly portable. 



Of the presence of pathogenic or disease-producing micro- 

 organisms in air, there is little or no direct evidence so far ; it 

 must, however, be remembered that it is just in the case of those 

 extremely infectious diseases, such as measles, whooping-cough,. 

 &c, in which the virus might be expected to be carried through 

 the air, that the exciting organized poisons have not yet been 

 discovered and identified. 



The investigations on aerial microbia, so far as they have as 

 yet been carried, are of service in indicating how we may escape 

 from all micro-organisms, whether harmful or harmless ; and 

 secondly, how we may avoid the conveyance of micro-organisms 

 into the atmosphere from places where pathogenic forms are 

 known or likely to be present. This acquaintance with the 

 distribution of micro-organisms in general, and the power of 

 controlling their dissemination which it confers, is really of far 

 wider practical importance than discovering whether some 

 particular pathogenic form is present in some particular sample 

 of air. It is this knowledge which has led to the vast improve- 

 ments in the construction and arrangement of hospital wards and 

 of sick-rooms generally, and which has directed attention to the 

 importance of avoiding all circumstances tending to disturb and 

 distribute dust. It is, moreover, this knowledge of the distribution 

 of micro-organisms in our surroundings which has formed one 

 of the foundations for the antiseptic treatment of wounds—that 

 great step in surgery with which the name of Sir Joseph Lister 

 is associated. 



1 "A New Method for the Quantitative Estimation of the Micro-organisms 

 present in the Atmosphere," Phil. Trans., vol. clxxviii. p. 113. 



