SOME NUCLEI OF CLOUDY CONDENSATION. 17 



say : " Apart from the explanation, the fact remains that by rapid and slow combustion 

 an agent is called forth which sets up condensation, and which does not disappear 

 immediately on the cessation of the originating process. Now so much oxidation 

 takes place in so many different ways, that the idea is at once suggested that this 

 agent which is formed by oxidation will be always present in small quantities in the 

 air. We consider it quite possible that the faint condensation which the steam jet 

 suffers in the open air may quite as well be due to the presence of this agent as to that 

 of dust." 



This conclusion seemed to be of sufficient importance to demand further investi- 

 gation. Admitting the existence of these " ions " in the products of combustion, the 

 question resolves itself into this : Do these " ions " persist for any length of time, and 

 so come to play some part in the condensation of a steam jet or of a cloud, or is their 

 action very short-lived? The difficulty of arranging an experiment for testing this point 

 is, that during combustion so many different products are thrown into the air as well 

 as an enormous number of fine dust particles. It is, therefore, difficult to find, by 

 testing this complex mixture, many of the constituents which cause cloudy condensa- 

 tion, whether any particular one is more affected by time than the others. Because, 

 if we keep the gases, new chemical combinations are formed, and the number of dust 

 particles also decreases, and it would be difficult to determine how much of the decrease 

 was due to loss of activity of the " ions." If, therefore, we are going to test the length 

 of time the " ions " of combustion retain their activity, we must use some form of 

 combustion in which no dust is formed, and the products are free from any substance 

 that can form on cooling solid or liquid nuclei. Hydrogen burned in oxygen seemed 

 particularly suitable for the purpose, but as this would involve considerable difficulties, 

 it was determined to try the effect of burning it in filtered air. It was, however, kept 

 in view that the presence of the nitrogen might give rise to complications, but fortu- 

 nately there was no trouble due to its presence ; nor did hydrogen peroxide, ozone, or 

 ammonium nitrate, which are said to be produced when hydrogen is burned, produce 

 any effect. Further, the products from hydrogen burning in air are stated by 

 Helmholtz to be very powerful in increasing the density of the steam jet, and they, 

 therefore, seemed the most suitable substance for an experiment of this kind. 



In carrying out this experiment the following arrangement was adopted (see Plate). 

 The hydrogen was prepared from zinc and sulphuric acid placed in the flask, A. After 

 being purified, it was burned at the platinum jet, B, having a diameter of 1 mm. The 

 air for combustion was drawn through the large cotton-wool filter, C, and brought in at 

 the lower end of the tall cylindrical glass combustion chamber, D, and taken off at the 

 top. The combustion chamber used for these experiments was a Fresenius Chloride of 

 Calcium Cylinder placed in an inverted position. For the present the flask, U, is 

 supposed to be removed, and the combustion chamber, D, connected directly by its 

 branch pipe, V, with the long glass tube, E, through which the products of combustion 

 are carried to the gasometer, F, by means of which the air and products of combustion 



