778 MAXIMUM DENSITY OF WATER. 



the liquid used is paraffine oil. Fig. 387 shows a con- 

 trivance which permits the heating of the paraffine oil 

 without danger. One branch of the bent tube is fixed 

 by means of a perforated cork so as to be 

 within a wider tube ; the narrower tube is 

 first filled to within a few centimetres from 

 the top with paraffine oil, and then the wider 

 tube is filled up to the mouth with hot water. 



The small funnel-shaped piece of glass broken off in 

 preparing the float (fig. 385) may be used for filling the 

 narrow tube with paraffine oil ; the use of a pipette for 

 the purpose is not advisable, as it is difficult to clean 

 it afterwards completely. A lamp cylinder, with its 

 wider opening upwards, may be used for holding the 

 hot water, if a straight glass tube of the requisite 

 width is not at hand. Such a cylinder is very liable to 

 crack when hot water is poured into it, but this may 

 be prevented by placing it in a large pan with cold 

 water, heating the water until it boils, and then allow- 

 ing the water to cool very slowly. 



^FIG. 387 As has been stated previously, mercury is 

 the only liquid which has been found to ex- 

 pand at a nearly uniform rate. Water when it is near 

 the freezing-point shows a peculiar and anomalous be- 

 haviour in this respect. Water at does not expand 

 when heated, but its volume becomes slightly smaller 

 until its temperature is 4; from this point, when the 

 heating is continued, it expands again, and its rate of ex- 

 pansion becomes greater and greater as its temperature 

 rises. It follows that a mass of water occupies the 

 smallest possible volume at 4, and that its specific 

 gravity is greatest at that temperature, or, as it is 

 usually expressed, water has its maximum density at 4. 

 The specific gravity of water at 4 is that taken as a 



