48 SCIENCE PRIMERS. [MATERIAL 



rain-water are concerned the order of nature is 

 constant. 



This however is by no means the same thing as 

 saying that the properties of water are always the 

 same. In fact the properties of the substance, water, 

 vary immensely according to the conditions to which 

 it is exposed ; but, under the same conditions, they are 

 the same, so that we may still say that, so far as 

 water is concerned, the order of nature is constant. 



31. Increase of Heat at first causes Water 

 to Increase in Volume. 



It has been seen that a certain weight of water 

 always has the same volume under the same con- 

 ditions. The most important of these conditions is 

 the heat or cold to which it is exposed. Water which 

 has stood for some time in a warm room becomes less 

 in volume, or contracts, if it is taken into a cool 

 place ; while its volume increases, or it expands, if 

 it is made hot. The same thing is true of quick- 

 silver, of spirit, and of liquids in general. A thermo- 

 meter is simply a small flask the bulb with a long 

 and narrow neck the tube rilled with as much mer- 

 cury or spirit as will rise a short distance into the neck. 

 If the liquid in the bulb is warmed, its volume is in- 

 creased and it overflows into the tube, increasing the 

 height of the column of liquid in the tube. If, on 

 the other hand, the liquid in the bulb is cooled, its 

 volume is diminished ; and, as it shrinks, the column 

 of liquid in the tube flows back into the bulb, and 

 the level of the top of the column is lowered. 



If a mark is made on the tube, or on a scale fixed 

 to it, at the point which the liquid reaches when the 



