196 



NATURE 



yOcc. 30, 1880 



in freezing, melting, and boiling. It was shown that 

 particles of water vapour do not combine with each other 

 to form a cloud-particle, but the vapour must have some 

 solid or liquid body on which to condense. Vapour in 

 pure air therefore remains uncondensed or super-satu- 

 rated, while dust-particles in ordinary air form the nuclei 

 on which the vapour condenses and forms fog or cloud- 

 particles. 



This represents an extremely dusty condition of the 

 air, as every fog and cloud-particle was formerly reprc- 

 sente;! by a dust-particle, which vapour by condensing 

 upon it has made visible. When there is much dust in 

 the air but little vapour condenses on eich particle, and 

 they become but little heavier, and easily float in the air. 

 If there are few dust specks each gets more vapour, is 

 heavier, and falls more quickly. 



These experiments were repeated with an air-pump, a 

 little water being placed in the receiver to saturate the 

 air. The air was then cooled by slightly reducing the 

 pressure. When this is done with unfiltcred air a dense 

 cloudiness tills the receiver, but when with pure air no 

 fogging whatever takes place, there being no nuclei on 

 which the condensation can take place. In this experi- 

 ment, and in the one with steam, the number of cloud- 

 particles is always in proportion to the dust present. 

 When the air is nearly pure and only a few dust-particles 

 present, then only a few cloud-particles form, and they 

 are heavy and fall like fine rain. 



The conclusions drawn from these experiments are : 

 (i) that whenever water vapour condenses in the atmo- 

 sphere it always does so on some solid nucleus ; (2) that 

 dust-particles in the air form the nuclei on which the 

 vapour condenses ; (3) that if there was no dust there 

 would be no fogs, no clouds, no mists,, and probably no 

 rain, and that the supersaturated air would convert evcr\' 

 object on the surface of the earth into a condenser on 

 which it would deposit ; (4) our breath when it becomes 

 visible on a cold morning, and every puff of steam as it 

 escapes into the air, show the impure and dusty condition 

 of our atmosphere. 



The source of the fine atmospheric dust was then 

 referred to, and it was shown that anything that broke up 

 matter into minute parts would contribute a share. The 

 spray from the ocean, when dried and converted into fine 

 dust, was shown to be an important source. Meteoric 

 matter also probably contributed a proportion. Attention 

 was then directed to the power of heat and combustion as 

 a source of this fine dust. 



It was shown that if there is much dust then each 

 particle only gets a little vapour condensed upon it, that 

 when the particles are numerous they become but little 

 heavier, and easily float in the air, and give rise to tint 

 close-packed but light form of condensation which consti- 

 tutes a fog, and therefore whatever increases the amount 

 of dust in the air tends to increase fogs, and that when 

 the dust-particles are not so nimierous the cloud-particles 

 are larger and settle down more quickly. 



It was shown that by simply heating any substance, 

 such as a piece of glass, iron, brass, &c., a cloud of dust 

 was driven off, which, when carried along with pare air 

 into the experimental receiver, gave rise to a dense fog 

 when mixed with steam. So delicate is this test for dust 

 that if we heat the one-hundredth of a grain of iron wire 

 the dust driven off from it will give a distinct cloudiness 

 in the experimental receiver, and if we take the wire out 

 of the apparatus and so much as touch it with our fingers 

 and again replace it, it will again be active as a cloud- 

 producer. Many different substances were tried, and all 

 were found to be active fog-prof ucers. Common salt is 

 perhaps one of the most active. 



Heat, it is well known, destroys the motes in the air, 

 and it might be thought that flame and other forms of 

 combustion ought to give rise to a purer air. Such how- 

 ever is not the case. Gas was burned in a gla'ss receiver. 



and supplied with filtered air for combustion, and it was 

 found that the products of combustion of pure air and 

 dustless gas gave rise to an intensely fog-producing 

 atmosphere. It may be mentioned Iiere that the fog- 

 producing air from the heated glass, metals, and burning 

 gas were each passed through the cotton-wool filter, and 

 the air was in all cases made pure, and did not give rise 

 to cloudiness when mixed with steam 



It will be seen that it is not the dust motes which are 

 revealed to us by a beam of sunlight when shining into a 

 darkened room, that form the nuclei of fog and cloud- 

 particles, as these may be entirely removed by heat, and 

 yet the air remain active as a cloud-producer. The heat 

 would seem to break up the larger motes which reflect the 

 light into smaller and invisible ones. When speaking of 

 dust, it is to these infinitesimally small and invisible par- 

 ticles we refer. The larger motes which reflect the light 

 will no doubt be active nuclei, but their number is too 

 small to have any important eftect. 



It is suggested, and certain reasons are given for sup 

 posing, that the blue colour of the sky is due to this fine 

 dust. 



Other experiments were made to test the fog-producing 

 power of the air and gases from different sources. The 

 air to be tested was introduced into the experimental 

 receiver and mixed with steam, and the relative densities 

 of the fog produced were noted. It was always found 

 that the air of the laboratory where gas was burning gave 

 a denser fog than the air outside, and that the air outside 

 varied, giving less fog during wet than during dry weather. 

 The products of combustion of gas burned in a Bunsen 

 flame, a bright flame, and a smoky flame, were all tested 

 and found to be about equally bad, and all much worse 

 than the air in which they were burned. Products of 

 combustion from a clear fire and from a smoky one gave 

 about equal fogging, and both much worse than the air 

 of the room. 



Experiments were made by burning different substances. 

 Common salt when burned in a fire or in alcohol flame 

 gave an intensely fog-produeing atmosphere, but burned 

 sulphur was the most active substance experimented on. 

 It gave rise to a fog so dense it was impossible to see 

 through a thickness of S cm. of it. 



The vapours of other substances than water were tested 

 to see if they would condense in the cloud form without 

 nuclei on which to deposit. All the substances experi- 

 mented on, which included sulphuric acid, alcohol, benzole, 

 and paraffin, only gave a cloudy condensation when mixed 

 with ordinary unfiltered air, and remained perfectly clear 

 when mixed with filtered air, all these acting Hke water 

 vapour. 



Before referring to fogs, which have now become so 

 frequent and aggravated in our large towns, it was pointed 

 out that caution was necessary in applying the results of 

 the experiments. 



The conditions of a laboratory experiment are so 

 diflerent, and on so small a scale, that it is not safe to 

 carry their teaching to the utmost limits and apply 

 them to the processes which go on in nature. We may, 

 however, look to the experiments for facts from which 

 to reason, and for processes which will enable us to 

 understand the grander workings of nature. 



It having been shown that vapour, by condensing on 

 the dust-particles in the air, gives rise to a fogging, the 

 density of which depends on the amount of fine dust in 

 the air ; the more dust the finer are the fog-particles, and 

 the longer they remain suspended in the air. It having 

 been also shown that all forms of combustion, perfect 

 and imperfect, are producers of fog nuclei, it is con- 

 cluded that it is hopeless to expect that, adopting more 

 perfect forms of combustion than those at present in use, 

 we shall thereby diminish the frecjuency, persistency, or 

 density of our town fogs. More perfect combustion will, 

 however, remove the pea-soup character from the fogs 



