SOME NUCLEI OF CLOUDY CONDENSATION. 23 



The experiments made with nitrous acid must be looked on with considerable doubt, 

 owing to the uncertainty which hangs round the existence of this gas when produced by 

 the ordinary methods. I shall, therefore, only describe the experiments made, and 

 leave the reader to draw his own conclusions. In experimenting with this gas it was 

 always made outside the test-flask, and drawn in through the cotton- wool filter. The 

 gas was prepared by means of nitric acid and arsenious anhydride. With gas prepared 

 in this way, it is probable some of the effect might be due to nitric acid passing over 

 with the nitrous acid. Another sample of nitrous acid was prepared from weak 

 sulphuric acid and sodium nitrate. Both these samples of nitrous acid gave dense 

 condensation after being exposed to sunshine. 



The hydrogen peroxide was tested by placing a little of the ordinary 10 per cent, 

 solution in the test-flask and sunning the contents. It proved to be a powerful 

 generator of nuclei. 



Sulphurous acid is one of the most puzzling of all the gases tested, its action is so 

 uncertain. Unlike the others, it almost always gives rise to condensation in the dark. 

 On some days, with even a weak solution in the test-flask, it was impossible to get the 

 condensation to cease entirely. When attempting to clear the air in the flask, even in 

 the dark, a few drops fell at each expansion, and on most days, though the drops nearly 

 ceased to appear, yet on standing a few minutes a considerable number made their 

 appearance when the air was expanded ; but in all cases the cloudy condensation was 

 very greatly increased after sunning. The products of combustion from household gas 

 in some cases gave rise to slight condensation after sunning, whilst the products of 

 combustion from anthracite coal, which probably contained a good deal of sulphur, 

 gave dense condensation after being exposed to sunshine. This test might possibly be 

 used for the detection of sulphur compounds in household gas. 



The sulphuretted hydrogen tested was prepared from sulphide of iron and weak 

 sulphuric acid. The gas was generated in a glass bottle, and drawn into the test-flask 

 through the cotton-wool filter. This gas gave dense condensation after being exposed 

 to sunshine. In order to see that the effect was not due to vapour of sulphuric acid, 

 the acid used was tested alone before adding the sulphide of iron, and was found 

 to be inactive. 



Vapour from hot hydrochloric acid was drawn into the test-flask through the filter 

 and sunned, after which it gave dense condensation on expansion, whilst the same 

 vapour was inactive whilst kept in the dark. 



Chlorine is one of the most interesting of the gases tested, as it caused condensa- 

 tion to take place without supersaturation. The gas was prepared from potassium 

 bichromate and hydrochloric acid ; it was drawn into the test-flask through the filter. 

 Whilst this gas gave no condensation on being expanded, if kept in the dark, it became 

 fogged on exposure to sunlight without being expanded, and the density of the con- 

 densation was but little increased by expanding it. 



A point of some importance connected with these nuclei due to the action of light 



VOL. XXXIX. PART I. (NO. 3). E 



