32 
POPULAR SCIENCE REVIEW. 
Any person wlio lias used the ordinary paraffin lamp oil, 4 Ran- 
goon oil/ or similar hydro- carbons as lubricators for machinery, 
must be well aware what a firm and coherent film they instantly 
form on the surface of water, and even on polished metal. In 
the latter case, their great preservative power against rust and 
tarnish is no doubt mainly due to this property — a property 
which has of late been further utilized in surgery for the pro- 
tection from the air of raw wounded surfaces in man and animals. 
To test the exact amount of the difference of evaporation, 
two platinum dishes containing water, and presenting equal 
surfaces of liquid, were placed side by side in a moderate 
draught of air, the water in one being coated with a very thin 
film of coal-tar. It was found that during twenty-four hours 
the evaporation was reduced by the film of coal-tar by 84’4 per 
cent in one case, and by 78*6 per cent in another. In order to 
imitate more nearly the action of actual smoke in foggy air, the 
smoke from burning coal was then blown on the sur- 
face of the water in one of the platinum dishes, and the 
evaporation was thus reduced by 77*3 per cent. In a large bell 
jar, the air of which was kept dry by a large surface of concen- 
trated sulphuric acid, the effect of a film of coal- smoke varied, 
in six experiments, from 66*6 to 92‘7 per cent. 
All these concordant results are much too large to be acci- 
dental or erroneous, the only possible conclusion being that the 
real though vague connexion between fog and the imperfect 
combustion of bituminous coal, if not absolutely demonstrated, 
has thus, to a great extent, been explained and accounted for. 
Dr. Frankland remarks incidentally that the presence of liquid 
hydro-carbons in a diffused condition would also tend to explain 
the frequency, persistency, and irritating character of the fogs 
which afflict our large towns, inasmuch as some of the products 
of destructive distillation of coals are very irritating to the 
respiratory organs. 
It has long been well known to practical men that we burn 
our ordinary domestic coal to a terrific loss. It is still more 
disheartening to think that we convert it into an interceptor of 
the light of heaven, and into a dangerous bronchial irritant. 
The worst of the outlook is that the evil seems to be spreading. 
‘ The fogs of the earlier portion of the present year were not 
confined to London. They formed belts which reached from 
Paris in the south to Liverpool in the north. The whole 
English Channel was shrouded in them. River traffic was 
stopped repeatedly by them in Liverpool. But it is none the 
less true that what in the country was a clear and comparatively 
harmless white fog, was in London a dirty and noisome fog/ 
And hereout comes a social paradox of no small dimensions, with 
whichDr. Siemens, more $wo,has not hesitated to grapple honestly. 
