1892.] 



connected ivith Cloudy Condensation. 



429 



by which it is connected with the experimental receiver. The stopcock 

 is closed, and the pump worked till most of the air is taken out of 

 the receiver ; and when it is desired to expand the air in the experi- 

 mental tube the stopcock is opened, when a violent rush of air takes 

 place, and the pressure is rapidly lowered in the experimental receiver, 

 and a dense colour-producing form of clouding is produced. 



The Conditions causing the Different Colours. 



For studying the conditions which give rise to the different colours 

 seen in these tubes, the air-pump and the small tube will be found to 

 be the most suitable. Supposing these to be fitted up, the following 

 will show how the colours change with the conditions : — 



First, the effect of the degree of saturation of the air. If the air be 

 dry the colours are not good, and some degree of expansion requires to 

 be made before any effect whatever is produced on the air ; and when 

 the colours do appear, it is only in the centre of the tube that they 

 are seen, the space, all round next the walls being free from condensed 

 particles. As the humidity is increased, this unclouded space near 

 the sides of the tube gradually diminishes. The colours are, there- 

 fore, best studied when the air is saturated and the inside of the tube 

 wet. When this is done the colours extend to the walls, and com- 

 pletely fill the tube. 



Second, the effect of the number of dust particles in the air. If 

 we use ordinary outside air, the colours are very faint or invisible. 

 Suppose some slight colour is visible, then it will be found that a 

 very slight expansion, say one-fifth of a stroke of the pump, will 

 give a pale blue, and if the expansion be increased the colour will 

 change. If, now, we use air from a room where gas is burning, and 

 fill the tube with it, we shall now, on expansion, get a much deeper 

 blue, and it will be observed that a greater expansion must now be 

 made to get the best blue, and before the colour begins to change. If 

 we alter the conditions still further, and fill the tube with air in 

 which is mixed a good deal of the products of combustion, we shall find 

 that the condensation is now so dense that we can scarcely see through 

 the tube ; but it will be noticed that the colour is a very deep blue, 

 and that a full stroke of the pump was necessary to produce this deep 

 blue, but in this case no change of colour was produced with that large 

 degree of expansion. 



These experiments show that, with few dust particles, a slight ex- 

 pansion will produce the best blue, and that, as the number of particles 

 increases, the amount of expansion necessary to produce the best 

 blue also requires to be increased, the depth of colour increasing 

 with the increase in the number of dust particles. The explanation 

 of the differences here is very simple. With few particles a very 



vol. lt. 2 a 



