474 



NATURE 



\_March 20, 1890 



a sunbeam, or a ray from the electric lamp, is of an organic 

 nature. It is seen to consist of countless motes, rising, falling, 

 or gyrating, although it is impossible to follow any of them with 

 the eye for longer than a fraction of a second. We conclude 

 that their weight exceeds but very slightly that of the air, and 

 moreover, that the atmosphere is the seat of multitudes of 

 minute currents, assuming all kinds of directions. Similar cur- 

 rents, though on a much larger scale, are also met with in the 

 air. One day last June, from the top of Eiffel's Tower in Paris, 

 I amused myself throwing an unfolded newspaper over the rail 

 carried round the summmit of the tower. At first it fell slowly, 

 carried away by a light breeze, but presently it rose, and, de- 

 scribing a curve, began again to fall. As it was vanishing from 

 sight, the paper seemed to me as if arrested now and then in its 

 descent, perhaps undergoing again a slight upheaval. Here 

 was, indeed, a gigantic mote floating in the atmosphere, and 

 subject to the same physical laws, though on a larger scale, as 

 those delicate filaments of dust we see dancing merrily in a 

 sunbeam. 



I recollect witnessing at one of the Friday evening lectures of 

 the Royal Institution in the year 1870 the following beautiful 

 experiment of Dr. Tyndall, illustrative of the properties of 

 atmospheric dust : — If we place the flame of a spirit-lamp or a 

 red-hot metal ball in the track of a beam of light, there will be 

 seen masses of dark shadows resembling smoke emitted in all 

 directions from the source of heat. At first sight this appears 

 as if due to the dust-particles being burnt into smoke ; but by 

 substituting for the spirit- flame or red-hot metal ball an object 

 heated to a temperature too low to burn the motes, the same 

 appearance of smoke is observed, hence the phenomenon is 

 not owing to the combustion of the dust. The explanation, 

 'however, is obvious. The source of heat, by warming the air 

 in its contact, and immediate proximity, made the air lighter 

 and the motes relatively heavier, consequently they fell, and 

 left spaces free from dust. These spaces in the track of the 

 electric ray appeared dark, or looked as if full of a dense smoke, 

 because the light of the ray could no longer be scattered in them 

 from the absence of dust. 



The motes were next examined by Tyndall, to determine 

 whether they were organic or mineral. This was done by 

 driving a slow current of air through a platinum tube heated to 

 redness, and examining this air afterwards in a beam of light ; 

 it was then found to darken the ray, having lost the power of 

 scattering light ; therefore the dust had been destroyed or burnt 

 lay passing through the red-hot platinum tube, clearly showing 

 its organic nature. 



We breathe into our lungs day and night this very finely- 

 divided dust, and yet it produces no ill effect, no bronchial 

 irritation. Tyndall has again shown by the analytical power of 

 a ray of light what becomes of the motes we inhale. 



Allow me to return to the experiment with the red-hot metal 

 ball placed in the beam of the electric light. Should a person 

 breathe on the heated ball, the dark smoke hovering around it 

 will at first disappear, but it will reappear in the last portions of 

 the air expired. What does this mean? It means that the 

 first portions of air expired from the lungs contain the atmo- 

 spheric motes inhaled, but that the last portions, after reaching 

 the deepest recesses in the organs of respiration, have deposited 

 there the dust they contained. 



It is difficult to say how much of the dust present in the air 

 may become a source of disease, and how much is innocuous. 

 Many of the moles belong to the class of micro-organisms, and 

 the experiment to which we have just referred shows how easily 

 these micro-organisms, or sources of infectious diseases, can 

 reach the lungs and do mischief if they should find a condition of 

 the body on which they are able to thrive and be reproduced. 

 Atmospheric motes, although it has been shown that they are 

 really deposited in the respiratory organs, do not accumulate in 

 the lungs and air-passages, but undergo decomposition and 

 disappear in the circulation. Smoke, which is finely-divided 

 coal-dust, is clearly subjected to such a destructive process ; 

 otherwise the smoky atmosphere of many of our towns would 

 soon prove fatal, and tobacco smoke would leave a deposit 

 interfering seriously after a very short time with the phenomena 

 of respiration. 



Dust, however, in its physical aspect is far from being always 

 innocuous, and, as you are aware, many trades are liable to 

 suffer from it. The cutting of chaff, for horses' food, is one of 

 the most pernicious occupations, as it generates clouds of dust of 

 an essentially penetrating character. Those engaged in needle 



manufactures and steel-grinders suffer much from the dust of 

 metallic particles. Stone-cutters, and workmen in plaster of 

 Paris, coal-heavers, cotton and hemp spinners are also engaged 

 in trades injurious to health because of the dust these men un- 

 avoidably work in. Those engaged in cigar and rope manu- 

 factures, or in flour-mills, hat and carpet manufacturers, are also 

 liable to suffer for the same reason. A number of methods have 

 been adopted, more or less successfully, to rid these trades of 

 the danger due to the presence of dust. I shall not detain you 

 on this subject, which would carry me too far, but merely bring 

 to your notice the fact I observed many years ago, that charcoal 

 has the power of retaining dust in a remarkable degree. I had 

 charcoal respirators made of such a form as to cover both the 

 mouth and nose, and containing about ^-inch thick of charcoal 

 in a granular state. I could breathe through such a respirator 

 in the thickest cloud of dust made by chaff-cutting without being 

 conscious of inhaling any of the dust. 



The subject of micro-organisms belongs to the science known 

 as micro-biology. As meteorologists we are chiefly concerned 

 with their distribution in the atmosphere. Micro-organisms are 

 dust-like particles capable of cultivation or reproduction in 

 certain media and at certain temperatures. If a particle of 

 matter known to contain micro-organisms, also called bacilli, be 

 placed on a clear surface of gelatine and maintained at a tem- 

 perature favourable to its development, in a short time the 

 gelatine will be found to contain a colony of those same bacilli. 

 A fact so often stated as to become a medical truism is that there 

 can be no infectious disease without the presence of the micro- 

 organism special to that disease. Open cesspools, putrid meat 

 or vegetable matter, accumulations of refuse, have no ill effects 

 on health unless the micro-organisms of a certain disease, as those 

 of typhoid fever or cholera, be present. On such foul decom- 

 posing matters these organisms thrive. They are reproduced 

 with great activity, and become virulent in their effects. 



Micro-organisms are scattered everywhere in the atmosphere. 

 Dr. Miguel, at the Montsouris Observatory at Paris, has made 

 an extensive inquiry into their distribution in air and water. 

 In this country Dr. Percy Frankland has, with praiseworthy 

 labour and perseverance, investigated the subject of micro- 

 organisms, and ascertained their number in various localities. 

 The result of his inquiry is that in cold weather, especially when 

 the ground is covered with snow, the number of organisms in 

 the air is very much reduced, and presents a very striking con- 

 trast with that found in warmer weather. The experiments 

 made on March 9 show that during cold and dry weather, with 

 a strong east wind blowing over London, a large number of 

 micro-organisms may still be present in the air. It is particularly 

 noticeable that even after an exceedingly heavy rain, and within 

 a few hours afterwards, the number of micro-organisms in the 

 air should be as abundant as usual. Taking an average of the 

 experiments made on the roof of the Science Schools of the 

 South Kensington Museum, the mean number of organisms 

 found in 10 litres of air amounted to 35, while an average of 279 

 fell on one square foot in one minute. Other experiments made 

 near Reigate and in the vicinity of Norwich present a marked 

 contrast with those undertaken in the South Kensington Museum. 

 There was a remarkable freedom from micro-organisms of the 

 air collected on the heath near Norwich during the compara- 

 tively warm April weather, when the ground was dry. The air 

 in gardens at Norwich and Reigate was richer in micro-organisms 

 than that of the open country. Again, the number of organisms 

 found in the air of Kensington Gardens, Hyde Park, and Prim- 

 rose Hill was less than in that taken from the roof of South 

 Kensington, but greater than in the country. 



Experiments made in inclo-ed places, where there is little 

 or no aerial motion, show the number of suspended organisms 

 to be very moderate, but as soon as any disturbance in the 

 air occurs, from draughts or people moving about, the number 

 rapidly increases and may become very great. Experiments 

 made in a railway carriage- afford a striking example of the 

 enormous number of micro-organisms which become suspended 

 in the air when many persons are brought together. 



Micro-organisms being slightly heavier than air, have an in- 

 variable tendency to fall, and on that account frequently collect 

 on the surface of water ; hence rivers, lakes, and ponds are 

 constantly being thus contaminated. Micro-organisms in very 

 pure water are not readily disposed to multiply, but traces of 

 decomposing organic matter will induce their reproduction. One 

 remarkable case occurs to me illustrating this fact. In 1884 a 

 severe epidemic of typhoid fever broke out in the town of 



