PARTICLES IN FOGS AND CLOUDS. 421 



Having diagnosed the disease a little closer, we seem to be in a better position for, if 

 not prescribing a remedy, at least suggesting a direction in which a mitigation of the 

 discomforts of our town fogs may be looked for. The object to be aimed at would appear 

 to be to alter the composition of the products of combustion in such a way that they shall 

 have no affinity for water vapour — so to change their composition that their present power 

 of condensing water may be destroyed, and they be converted into some kind of matter 

 indifferent to it. So far as we see at present, it is on this field that the battle must be 

 fought. It seems possible that the present water- attracting products of combustion 

 might be changed, and their evil influence destroyed, by the addition to the coal of 

 something which, when burned along with it, will produce this result. This is a question 

 to which we wish to direct the attention of chemists, and it is to be hoped some sub- 

 stance may be found which will produce this effect, and admit of being burned along 

 with the coal, or of being added to the air of our towns, and the persisting nature of 

 town fogs destroyed. 



As is well known, the products of combustion of different substances have very 

 different fogging effects. Any experiments which I have been able to make in this 

 direction are, however, on too small a scale to be of value. In a question of this kind, 

 laboratory experiments are apt to be misleading. To be of value, they ought to be con- 

 ducted on as large a scale as possible. I may, however, mention here the result obtained 

 in laboratory experiments with a few substances, as illustrative of the effects of the 

 different kinds of nuclei produced by the combustion of different substances. The method 

 of testing the fogging powers of the nuclei was the same as that described in this paper 

 to illustrate the difference between town and country fogs. The air of the room was 

 admitted to one receiver, and the products of the combustion we wished to test to the 

 other. The contents of the two receivers were then quickly and simultaneously 

 expanded. Any difference in the density of the condensations in the two receivers gave 

 the effect of the products of combustion on the air of the room. If it were desired to 

 get the comparative effect of different products, they were admitted to different receivers, 

 the products being collected from flames of as nearly as possible the same size. As the 

 receivers were both reduced to the same pressure, the same proportion of products was 

 admitted to each. The density of the condensation by this method of testing gives a 

 rough indication of the comparative increase in the number of nuclei produced by the 

 combustion of the different substances. 



To test the persistence of the fogs formed by the products of combustion of different 

 substances, two methods were employed : — 1st, by observing the fogs produced by rapid 

 expansion, and noting the comparative rates at which the fogs in the different receivers 

 fell, the degree of clearness in the air over them, and the rate at which the air cleared in 

 the lower part of the fog ; 2nd, by examining the amount of fog formed in the different 

 products after being admitted to the receiver, and time given for them to become 

 saturated with water vapour, but without being expanded. This examination was made 

 by means of a beam of light from a dark lantern, the light being brought to a focus by 



VOL. XXXVII. PART II. (NO. 20). 3 R 



