.SOME NUCLEI OF CLOUDY CONDENSATION. 21 



is nearly freed of all nuclei of condensation by the electric discharge, which, while it 

 deposits the dust, rather increases the " ions." It may also be remembered that air 

 passed through the Thermic filter is freed of all nuclei. 



But whilst all experimental tests indicate that these " ions," or whatever it is that 

 is produced during combustion, and which gives rise to condensation, are extremely 

 short-lived, and do not play any important part in cloudy condensation in the 

 atmosphere, it would be rash to say that they never do play any part. The conditions 

 of combustion of a laboratory experiment are different from those under which com- 

 bustion usually takes place ; and further, whilst hydrogen may not give any long-lived 

 " ions," some other forms of combustion may, and though they lose their power of 

 discharging electrified bodies, they may yet be capable of causing cloudy condensa- 

 tion. 



From many hundreds of observations on the dust and the transparency of the 

 atmosphere, it has been shown that, for the same relative humidity, the transparency is 

 proportional to the number of dust particles in the air, except at one place of observa- 

 tion, and under exceptional conditions. # Now this is not likely to be the case if " ions " 

 played any important part in the condensation, as we can hardly expect their action 

 to bear a constant proportion to that of the dust. For the reasons above given, it 

 therefore does not appear at all likely that " ions " play any important part in the 

 ordinary cloudy condensation in the atmosphere. 



PART II. 

 Sunshine and Cloudy Condensation. 



In 1894 there was communicated to this Society Part III. of a paper "On the 

 Number of Dust Particles in the Atmosphere." It is there shown that under certain 

 •conditions the sun gives rise to a great increase in the number of nuclei. The observa- 

 tions made at Kingairloch show, that when the wind is from the N.W. the number 

 of particles is always very low, being generally under 500 per c.c, and often very 

 much lower when the sky is clouded, but that when the sun came out the numbers 

 rapidly increased as the day advanced, and if the sun kept shining all day, the numbers 

 rose to many thousands in the afternoon, after which the numbers decreased, and 

 again were very low next morning if the wind continued to blow from the N.W. 

 These observations entirely justify the prediction which was made in 1881 on the 

 probable action of sunshine in producing nuclei of condensation. 



This action of sunshine on our atmosphere evidently deserves further investigation. 

 A few of the ordinary constituents and impurities of our atmosphere have therefore 



* Proc. Roy. Soc, Edin., vol. xx. pp. 66, 93 ; Trans. Roy. Soc, Eclin., vol. xxxvi., Part III., pp. 621, 693. 



