OBJECTS.] INTRODUCTORY. 75 



diffused into the water for a short distance, but not far, 

 showing that only a slight mixture is taking place. 

 This, however, is not because the two fluids mingle 

 with difficulty ; for, with slight stirring, they mix com- 

 pletely, and you have a fluid the colour of which is 

 about half as intense as that of the alcohol, and many 

 of the other properties of which are intermediate 

 between those of pure alcohol and those of pure 

 water. 



Thus far, nothing further than simple mixture, as 

 when coloured water was added to pure water, seems 

 to have occurred ; but, in reality, something more has 

 happened. In the first place, the mixture is a good 

 deal warmer than either of its components ; that is to 

 say, heat has been generated. In the second 

 place, if you measure the volume of the whole fluid 

 after it has cooled, it no longer stands at the mark 

 ten but distinctly lower, or about nine and three- 

 quarters. As the volume of the mixture is less 

 than the sum of the volumes of its two components, 

 it follows that the density of the mixture must 

 be greater than a density midway between that 

 of the water and that of the alcohol. In other 

 words, the molecules in the mixture do not occupy 

 the same space as they did when they were sepa- 

 rate. The result is the same as if the ten volumes 

 had been compressed until they occupied only nine 

 and three-quarters ; so that the effect is a contraction 

 similar to that which would be brought about by 

 taking away heat from the mixture. In fact, as we 

 have seen, the mixture gives out a quantity of heat. 



There is another respect in which the mixture is 

 unlike both its constituents. It both boils and 



