22 MR JOHN AITKEN ON 



been tested to see if sunshine acted on them in such a way as to make them active as 

 producers of nuclei of cloudy condensation. 



The gases tested were ammonia, nitric acid, nitrous acid, peroxide of hydrogen, 

 sulphurous acid, sulphuretted hydrogen, hydrochloric acid, and chlorine. The result of 

 these tests is that the gases or vapours of all these substances give rise to nuclei of 

 condensation after being acted on by sunshine. 



The apparatus used for these experiments consisted of a glass flask, in which the 

 gases were sunned and tested. This flask was provided with an air-pump and a cotton- 

 wool filter, the arrangement being the same as that shown on the right-hand side of 

 the Plate, with the centre tube removed. Different methods were used for getting the 

 gases into the flask ; in some cases a little of their solutions in water was introduced 

 into the flask, in others the gases were drawn into the flask through the cotton-wool 

 filter. 



In making a test, the first thing to be done was to wash the flask and connecting 

 tubes, and put new cotton-wool into the filter. Filtered air was then pumped in till 

 no drops appeared on expanding the air. If a solution of any of the gases was in the 

 flask, the apparatus was then removed to an open window and exposed to sunshine, 

 generally for one minute, after which it was brought back, the air expanded by means 

 of the pump, and the density of the condensation noted. If the gas was introduced 

 into the flask through the filter, it was sometimes necessary to pass a considerable 

 quantity of vapour when making the first test, as it was found that the vapour that 

 first entered the filter did not pass through, but seemed to combine with the cotton- 

 wool, or perhaps only condensed on it, as after a filter was saturated it acted for some 

 time, supplying nuclei without a fresh quantity of vapour being passed into it. 



I regret that the experiments on the action of sunlight on the vapours mentioned are 

 far from being as complete as I could wish ; but for the reasons already given, and on 

 account of the difficulty of getting plenty of sunlight, it has not been found possible to 

 get the work done. The following notes are, therefore, very incomplete. 



It should be noted at the outset that ordinary air, after being filtered and exposed 

 to sunshine, does not show any cloudy condensation on expansion, only occasionally a 

 drop or two may appear ; but when any of the above-named gases are in the air a very 

 different result is obtained. Ammonia has been experimented with both by placing a 

 weak solution in the test-flask, and by drawing the vapour into the flask through the 

 filter. If one drop of ammonia be added to 100 c.c. of water, and put in the flask 

 and sunned for one minute, a great deal of condensation takes place on expansion with 

 even this very weak solution, and if we expose it for five minutes the condensation 

 is much more dense. If we put a weak solution of nitric acid in the flask, say two 

 drops to 100 c.c. of water, we get cloudy condensation on expansion after being sunned ; 

 but the action of nitric acid is not so powerful as ammonia ; that is to say, that though 

 the solution of nitric acid was double the strength, it did not give so dense a con 

 densation as the vapour from solution of ammonia. 



