76 SECRETS OF EARTH AND SEA 



Vapours are given off by many liquids, and even by 

 some solids, varying in their production according to the 

 heat applied in different cases. They are gases and quite 

 transparent and invisible at the proper temperature, like 

 the atmospheric air. Thus water is always giving off 

 "water-vapour," which is quite invisible. When water 

 is heated to the boiling point it is rapidly converted 

 into transparent invisible vapour. Steam, as this vapour 

 is called, is invisible, and we all habitually make a 

 misleading use of the word " steam " when we apply it 

 both to this and to the slightly cooled and condensed 

 cloud which we can see issuing from the spout of a 

 kettle or from a railway engine. It seems that the 

 fault lies with the scientific writers, who have applied 

 the word " steam " to the invisible water vapour or gas 

 before it has condensed to form a cloud. The old English 

 word " steam " certainly means a visible cloudy emana- 

 tion, and not a transparent invisible gas. A cloud is not 

 a vapour, but is produced by the coming together or 

 condensation of the minute invisible particles of a vapour 

 to form larger particles, which float and hang together, 

 and reflect the light, and thus are visible. 



By the examination of other vapours or gases than that 

 which is gaseous water, namely, the vapours of bodies like 

 chloroform and ether, it has been shown that " cloud " 

 forms in a vapour not merely in consequence of the 

 cooling of the vapour, but in consequence of the presence 

 in the air (or in the tube in which the vapour is enclosed 

 for observation) of very fine floating dust particles. They 

 act as centres of attraction and condensation for the 

 vapour particles. When there are no dust particles 

 present clouds do not form readily in cooling vapours, 

 or only at lower temperatures, and in larger mass. 

 Tyndall made some beautiful experiments on this subject, 

 obtaining clouds of great tenuity in vapours enclosed in 



