204 



HEAT. 



FIG. 118. Bunsen's 



Raising of 



this fall agreeing very closely with that predicted by Prof. James 



Thomson. Dewar (Proc. R.S., xxx., 1880), using apparatus like that of 

 Cailletet for the liquefaction of gases, found that up 

 to 700 atmospheres the reduction was proportional to 

 the pressure and at the rate of 0-0072 per atmosphere. 

 Experiments have also been made by Bunsen and 

 Hopkins. Bunsen used a bent tube (Fig. 118), con- 

 taining a small quantity of the substance to be 

 tested at the end of the shorter limb OD, A^hich was 

 closed after its introduction. Mercury extended from 

 D round the bend, filling the wider part E of the 

 tube, and rising some distance in the capillary tube 

 AB. The end A was closed when the quantity of 

 mercury was suitably adjusted. The mercury, on 

 heating, expanded and compressed the air which 

 served both as a manometer and as a spring to exert 

 pressure. This pressure and the melting point when 

 it was exerted could be observed. Bunsen found that 

 100 atmospheres raised the melting point of sperma- 

 ceti about 3'2, and that of paraffin about 3'6. 



Hopkins, using an iron cylinder in which the 

 5 melting was indicated by the fall of a ball of iron 

 the within the cylinder, was able to put on greater pres- 

 Melting Point by sures, raising the melting point of spermaceti, for 

 Pressure. instance, from 51 to 80'2 by a pressure of 792 



atmospheres a result nearly agreeing with Bunsen's. 

 Melting Points Of Solids. Various methods have been used to find 



the melting points of solids. The simplest is to employ an arrangement 



similar to that used for fixing the zero point of thermometers. Heat 



is very gradually supplied to a mixture 



of the liquid and solid, and the steady 



point observed. The melting point of 



a substance like paraffin may be de- 



termined by filling the capillary part 



of a bent tube (Fig. 119), with the 



substance when liquid, freezing it, and 



then immersing the tube in a bath, 



and noting the temperature at which 



melting occurs. Carnelly determined 



the melting points of many salts which 



melt at a high temperature, by heating 



a small quantity in a platinum crucible 



of mass large compared with that of 



the salt, and then, at the moment of 



melting, dropping the crucible into 



a calorimeter. The quantity of heat 



given up allowed the temperature at FIG. 119. Melting-Point Apparatus. 



melting to be calculated from the 



known specific heat of platinum the heat given up by the salt being 



negligible. 



Latent Heat. The latent heat of many solids may be found by 



