422 MR JOHN AITKEN ON THE 



means of a condensing lens. The amount of the fog so formed by the products in moist 

 air indicated the tendency of the nuclei to form, dense and persisting fogs. All these 

 tests are comparative, either between the fogging of two products of combustion or 

 between one of them and the air of the room, which has always to be considered in such 

 experiments. 



Tested in the manner above described, alcohol gave very different results according 

 to the way it was burned. If the products of combustion from alcohol burned at a 

 platinum burner, or in an open vessel and without a wick, were admitted to one receiver, 

 and the air of the room to the other, and then rapidly expanded, there was scarcely 

 any difference in the density of the fogging in the two receivers. This result is interest- 

 ing from the fact that it is the only form of combustion yet observed which does not 

 greatly increase the number of nuclei. A very different result is obtained if we repeat 

 the experiment, and if in place of using a platinum burner, we use an ordinary wick, or 

 burn the alcohol in any way in which it shows the usual yellow sodium colouring. Com- 

 paring the products from such a flame with the air of the room, it will be found to give 

 a fogging many times more dense, showing that the slight difference in the flame makes 

 a very great difference in the number of nuclei produced by the combustion, due probably 

 to the presence of extremely minute particles of some sodium salt. But though the 

 ordinary alcohol flame gives rise to a great increase in the number of nuclei, yet the 

 particles produced by that form of combustion have but little affinity for water vapour, 

 and fogs formed of them clear rapidly ; they do not, therefore, form persisting fogs. 



An ordinary wax-candle gives only a slight increase in the number of nuclei, and 

 they have no condensing power. Matches, both wax and wooden ones, gave a great 

 increase in the number of particles. It seems strange that a wax-vesta should 

 give more nuclei than a candle, — that is, of course, using equal flames for equal times. 

 This difference probably results, as in the case of alcohol, from the impurities in 

 the wick, much of which is burned in a wax-match compared with a candle. Another 

 possible source of the great number of nuclei given by the match may be the chemicals 

 used for igniting them, as I find that if a match be ignited in a receiver, wetted inside, 

 the products of the combustion of the head of the match give rise to a very dense and 

 persisting form of fog without cooling by expansion, while if burned in dry air there is 

 only a very thin smoke. It therefore seems possible that some of these impurities may 

 continue to be given off while the match is burning. 



Gas, whether burned as a luminous or non-luminous flame, gives rise to a great 

 increase in the number of nuclei. An ordinary paraffin-lamp gives an increase similar to 

 that of gas. But in these cases there is but little affinity between the nuclei and water 

 vapour ; and fogs formed of them rapidly differentiate, and they have but little 

 tendency to cause fogging unless the air tends to be supersaturated. 



Many proposals have of late been made to prevent town fogs by the more perfect com- 

 bustion of the coal in household fires, furnaces, &c. There can be no doubt that perfect 

 combustion would do much to diminish the smoke element in town fogs, but it cannot 



