PARTICLES IN FOGS AND CLOUDS. 423 



tend to diminish the density of the fogs as fogs. To effect this something more that 

 perfect combustion is necessary. If we were to burn alcohol, paraffin, or purified gas 

 in our fires, we would get products which would have no tendency to produce persisting 

 fog particles ; but whenever we use coal in which there is the least sulphur, perfect com- 

 bustion will not prevent the products forming persisting fog particles. 



The following experiments may be made to illustrate this point : Prepare a good fire 

 and let it burn till it is perfectly bright and clear to the top, every indication of smoke 

 gone, and nothing rising from it but the pale blue flame of a clear fire ; or use a stove 

 burning char and also red to the top. Let us now draw some of the products of 

 combustion into a large dry flask and examine them to be sure there is no smoke. Being 

 satisfied that the products are clear, some of the same products are drawn into another 

 flask, but in this case wetted inside, or we may add water to the contents of the flask we 

 tested dry. If we examine the products tested in either of these ways, we shall find the 

 air in the interior of the flask to be thickly fogged. The path of a beam of light directed 

 through the flask will shine out brightly. As this condensation takes place without the air 

 being expanded, it indicates the presence of a number of nuclei having an affinity for 

 water vapour. It may be mentioned that in some cases, owing to the large proportion of 

 products compared with air in the receiver, dense fogs have been formed ; fogs which, if 

 they had been on a large scale, would have far exceeded the density of any town fog. 

 These fogs have also been seen to persist for hours, showing the presence of a large number 

 of particles having an affinity for water vapour. It was noticed that the products from 

 different parts of the fire gave fogs of very different densities. In all cases, if the airs 

 containing these products were expanded, they gave very dense fogs, showing a great 

 increase in the number of the particles due to the combustion of charcoal. 



In making these experiments one must be careful as to the manner of collecting the 

 gases. Precautions should be taken that the collecting pipe does not become highly 

 heated, or the products of combustion might be contaminated by impurities thrown off 

 by the metal. The simplest plan is first to heat the collecting pipe to a much higher 

 temperature than it will be exposed to while collecting the gases. In this way any 

 impurities that may be attached to the pipe are driven off. In using the pipe it should 

 be first cooled, then placed at a distance from the hot coals, but in the current of hot 

 gases, and removed before it has time to become highly heated. 



It should be remembered that there is always some ammonia in the atmosphere, and 

 it has been shown in a previous paper* that the presence of that substance in the air 

 greatly increases the fogging power of the products of combustion of sulphur. Dr W. J. 

 Russell has shown in a paper " On the Impurities of London Air " t that there is more 

 ammonia in town than in country air, and also that in towns there is more during foggy 

 weather than at other times. If we repeat the experiments above described with the 

 products from a clear fire, and add a little vapour of ammonia to the air in the flask, we 



* Trans. Roy. Soc. Edin., vol. xxx. part i. 



t The Monthly Weather Eeport of the Meteorological Office for August 1885. 



