November 5, 1891] 



MA rURE 



1 1 



surface" ; that as long as a molecule of liquid water is 

 surrounded by liVe molecules, and the same with gaseous 

 water, we do not know at what temperature, or whether at 

 any temperature, they would change their state ; but if in 

 contact with a solid then at the surface, where they meet, 

 the change will occur ; if the solid be ice it may become 

 liquid or the liquid may become solid, and the same kind 

 of action occurs when the liquid is in contact with its 

 own vapour ; in fact, that what we call the freezing and 

 boiling-points of a body are the temperatures at which 

 these changes take place at such free surfaces. The dust 

 always present in the atmosphere offers this free surface 

 to the gaseous water, and thus induces its condensation. 

 This specific action of dust varies very considerably, first 

 with regard to its composition, and second with regard to 

 the size and abundance of the particles present. Sulphur 

 burnt in the air is a most active fog-producer, so is salt. 

 Many hygroscopic bodies form nuclei having so great an 

 affinity for water that they can cause its condensation from 

 an unsaturated atmosphere. At the same time non- 

 hygroscopic bodies, such as magnesia and many others, 

 are powerful fog-producers ; no doubt their activity may 

 in part be attributed to their being good radiators of heat, 

 and thus becoming cooled, induce condensation. Mr. 

 Aitken also shows that the products of combustion, even 

 when the combustion is perfect, are powerful fog-pro- 

 ducers, a fact which has important bearing on the pro- 

 duction of town fogs. One other point in these experi- 

 ments I cannot omit mentioning, it is the exceedingly 

 minute amount of matter capable of inducing fog. In 

 his first "series of experiments Mr. Aitken showed that 

 ^ Jjj of a grain of iron wire, however often it was heated, 

 evolved on each heating sufficient dust to cause a visible 

 fog, and afterwards,with still more delicate apparatus, that 

 To'oo of ^ grain of either iron or copper, when treated in 

 the same way, gave a similar result, and from these last 

 experiments he infers that even jooVao grain of either 

 wire, if only slightly heated, would yield sufficient nuclei 

 to cause a visible amount of fog. It is of much impor- 

 tance and interest, seeing how small a cjuantity of dust is 

 required to produce fog, to know thac even this small 

 amount may be filtered out of the air by passing it 

 through cotton wool, and thus an air be obtained in 

 which a fog cannot be produced. Mr. Aitken's description 

 of such an atmospheie is at first most alluring, for he 

 says, if there was no dust in the air there would be no 

 fogs, no mists, and probably no rain ; but he goes on to 

 state that when the atmosphere became burdened with 

 supersaturated vapour, it would convert everything on the 

 surface of the earth into a condenser ; every blade of grass 

 and every branch of a tree would drip with moisture de- 

 posited by the passing air ; our dresses would become 

 wet and dripping, and umbrellas useless ; but oar miseries 

 would not end here, for the inside of our houses would 

 become wet, the walls and every object in the room would 

 run down with moisture. I think, if we picture to our- 

 selves this state of things, we may be thankful that there 

 is dust and fog. Dust in its finer forms is invisible to 

 us ; but as its delicate particles become loaded with mois- 

 ture, it becomes a fine mist, dense if the number of 

 particles are many ; if, however, the dust-particles are 

 fewer, and the amount of aqueous vapour the same, 

 each particle will have a larger amount of condensed 

 moisture to carry, and it will give rise to a more coarse- 

 grained fog ; the moisture, or some of it, will be more 

 feebly attached to its nuclei, producing then what is 

 known as a wjt fog, whereas at least a most important 

 fact in the production of a dry fog is the strong affi- 

 nity between the nuclei and the condensed vapour. As 

 most of you are no doubt aware, Mr. Aitken has invented 

 a most ingenious method for counting thenumber of dust- 

 particles in air, and has obtained most interesting and 

 valuable results. 1 can only mention here that some of 



NO. I 149, VOL. 4.5] 



these results deal with the clearness of air in relation to 



the number of dust-particles present, and other results 

 show how little effect rain has in diminishing the amount 

 of the finer dust in air. Evidently towns will supply dust 

 of all kinds, and therefore offer every inducement for 

 fogs to form there, and that at least some of the particles 

 will be capable of causing the condensation of moisture 

 even from an atmosphere which is not saturated with 

 aqueous vapour. This condensation of moisture is a very 

 complete process for removing all kinds of impurities from 

 the air. Floating particles are free surfaces, and become 

 weighted by the moisture they condense and tend to sink, 

 and even the gaseous impurities in the air will be dis- 

 solved and carried down by the moisture present. 



Experiment confirms this, for it has been proved how 

 correctly the impurities of an air can be ascertained by 

 determining the composition of dew, even if it be artifi- 

 cially and locally formed. It is of importance to note 

 that even the purely gaseous emanations from our towns 

 cannot pass away when a fog exists, as is shown by the 

 accumulation of carbonic acid which takes place during 

 a fog. Taking 4 volumes in 10,000 volumes as the normal 

 amount of carbonic acid in London air, some years ago I 

 found that it increased in the case of a dense fog to as 

 much as I4"i volumes, and often to double the normal 

 amount, which must represent a very serious accumulation 

 of the general impurities in the air. 



A fog in this way becomes a useful indicator of the 

 relative purity of the atmosphere in which it forms. If 

 pure aqueous vapour be condensed it gives a white mist — 

 a country fog, a sea fog— and a white light seen through 

 it is not converted into a red light ; but in town fogs the 

 whiteness of pure mist disappears and becomes dark, in 

 some cases almost black in colour, the change being pro- 

 duced by the foreign matters floating in the air, and by 

 far the most abundant colouring matters of our town fogs 

 are the products generated by the imperfect combustion 

 of coal ; but in addition to these bodies, many others must 

 obviously find their way into the air over a town. 

 Especially will there be dust from the universal grinding 

 and pounding going on in street traffic and many 

 mechanical operations, from the general disintegration 

 of substances and the decomposition of perishable 

 materials — all will add something to the air, and it will 

 become an integral part of the fog. However, although 

 it is often said that a town fog is so dense that it may be 

 cut with a knife, still it is difficult to condense so much of 

 it that it can be subjected to a searching chemical 

 analysis. In 1885, by washing foggy air, I was able to 

 determine the amount of sulphates and chlorides present, 

 and as indicators of organic matter the quantity of carbon 

 and nitrogen in the fog. The results showed strikingly 

 how largely the amounts of organic matter and ammonia 

 salts in the air varied with the weather ; no case cf dense 

 fog occurred when the experiments were being made ; but 

 the mean of several experiments cle.irly showed ihat in 

 foggy weather the amount of organic matter was double 

 as much as existed in the air in merely dull weather, and 

 that the amount of sulphates and chlorides increased 

 under like conditions, but not to the same extent. P'og 

 may, however, be made to give its own account of its 

 constituents, for we have only to collect and analyze the 

 deposit which it leaves to learn what its more stable con- 

 stituents are, and we have to thank the air-analysis com- 

 mittee of the Manchester Field Naturalists' Society for 

 the most complete analysis of such a deposit which has 

 yet keen made. The deposit analysed occurred during 

 ; the last fortnight in February ofthis year (1891), and was 

 j obtained from the previously washed glass roo.^s of the 

 I plant-houses at Kew, and Messrs. Veitch's orchid-houses 

 I at Chelsea. At Kew 20 square yards of roof yielded 30 

 grammes of deposit. At Chelsea the same area gave 40 

 ' grammes, which represents 22 lbs. to the acre or 6 tons 



