FOGS. 215 



luminous bar, which, on looking more closely, we find, is pro- 

 duced by the dust-motes floating in the air of the room reflecting 

 the light and becoming visible as they pass through the path of 

 the beam. And here is the answer to the second question : Are 

 these the fog-producers ? No. When air containing this dust is 

 heated or passed through a flame, these motes disappear and the 

 path of the sun becomes invisible. From this we might conclude 

 that air which passes over or through a flame where combustion 

 was perfect ought to be nearly dustless, and therefore ought not to 

 be a good medium for fogs. But we find air that has passed through 

 a flame is a good medium. What really happens appears to be 

 this : — The heating of the air by the flame increases the number 

 of particles. The heat would seem to destroy the light-reflecting 

 power of dust by breaking the larger motes into smaller ones, and 

 by carbonising or in some way changing their colour, and thus 

 making them less light-reflecting. 



The fact that the products of combustion are fog-producers 

 may be proved in the following way : — Light a gas-flame within a 

 glass tube, and allow the products of combustion to pass into a 

 vessel into which steam can be blown when required. 



The result of an experiment on these lines is that the com- 

 bustion of filtered air and dustless gas gives an extremely fog- 

 producing atmosphere. That the fogging is, however, due to dust 

 cannot be doubted, from the fact that the products of combustion, 

 when ^Ifered, give no cloudiness on the admission of steam. 



By further experiments, it has been shown that the products of 

 combustion from a clear or smoky fire give about equal fogging, 

 and that of all the substances experimented upon sulphur was the 

 most active fog-producer. In fact, sulphur vapour gives rise to a 

 fog so dense that it is impossible to see through a thickness of two 

 inches, a degree of opacity not yet reached by our London fogs. 



From these results, we learn that it is hopeless to expect that 

 by the adoption of fires having a perfect combustion — such as the 

 gas-fires, now so much advocated — we should diminish the 

 frequency, persistency, or density of our town fogs. All fires, 

 however perfect the combustion, are fog-producers when accom- 

 panied by certain conditions of moisture and temperature. From 

 this it will be seen that it is not the visible dust-motes seen in the 



