776 SPECIFIC GRAVITY ALTERED BY TEMPERATURE. 



Now, in this case the specific gravity of the cold water 

 is greater than that of the float, while when the water 

 is hot its specific gravity is less than that of the float ; 

 but since both bodies shortly after the float is immersed 

 are at the same temperature, we see that the decrease 

 in the specific gravity of the water is greater 

 A than that in the float, and that consequently 

 within the same range of temperature the ex- 

 pansion of the water has been greater than 

 that of the float. 



The float is made of a small test-tube, which is drawn 

 out to a point over a spirit or gas flame. The tube may 

 be much better handled during the operation, and it 

 may be drawn out very near the mouth without burning 

 the fingers, if a moderately tight-fitting cork is pushed a 

 few millimetres into the tube, so as to serve as a handle 

 FIG. 385 for turning the tube while it is heated. A small 

 Q real size), groove must be formed lengthways in the cork by 

 two slanting cuts with a sharp knife, so as to allow the air to ex- 

 pand when the tube is heated ; otherwise the confined air would 

 blow out the glass into a bulb as soon as it became soft by the 

 heat. 



A small flame should be used for the heating, and the tube turned 

 at a very uniform rate, without pulling the glass ; for the part 

 which is softened by the heat becomes narrower while the walls 

 at the same time become thicker : if the glass were drawn out at 

 once without allowing it to thicken, it would become much too thin 

 and brittle. After the glass is drawn out to the desired width and 

 has become cool a fine cut is made with a very sharp file, which is 

 somewhat moistened, and the float is then broken off. It is filled 

 with water, by means of a glass tube drawn out to a long narrow 

 point, so that it sinks in cold water to within about 5 mm or 6 mm 

 from the top. If too much water has been put in it is simply 

 shaken out by holding the float with the aperture downwards. 

 When the quantity of water is properly adjusted the tube is closed 

 by holding it vertically, and directing the end of the blowpipe flame 

 upon the extreme point of the glass, which must be quite dry ; any 

 water left within the aperture may be sucked out by means of the 

 long narrow glass tube. 



