HIGH-PRESSURE RESEARCH BRIDGMAN 159 



on melting, the latent heat of melting, and in many cases the differ- 

 ence of compressibility between liquid and solid. The effect of pres- 

 sure on the polymorpliic transitions of some 30 substances has been 

 investigated, including organic and inorganic compounds and the very 

 interesting case of water. This investigation has involved the study 

 of 69 transition lines and 21 triple points. On these transition lines 

 the change of transition temperature with pressure has been deter- 

 mined, and also the change of volume, the latent heat, the difference 

 of compressibility (which gives information about the difference 

 of thermal expansion and specific heats), and the effect of pressure 

 on velocity. In addition to these 30 substances, about 100 others 

 have been examined and no new polymorphic forms have been found. 

 The effect of pressure on electrical resistance over a range of tem- 

 perature has been determined for 45 metallic elements in the solid 

 state, 8 in the liquid state, and for 7 alloys. The effect of pressure 

 on thermal e. m. f. has been measured for 18 metallic elements and 2 

 alloys. The effect of pressure on the thermal conductivity of 11 

 solid metallic elements, 6 minerals, and 15 liquids (all organic except 

 water) has been measured. Finally, a few observations have been 

 made on the phenomena of rupture under high pressure. 



What now is the general nature of the facts found, and some 

 of their implications? First, consider the facts with regard to 

 changes of volume. At high pressures there is no essential differ- 

 ence between a gas and a liquid. Under 12,000 kg. the changes of 

 volume of ordinary nonmetallic liquids range from something like 

 20 per cent for water and 33 per cent for ether up to those of 

 hydrogen and helium, which are the most compressible substances 

 investigated, and which, under 12,000 kg. at ordinary temperatures 

 have a volume less than half that which they have when frozen 

 solid at atmospheric pressure by temperatures close to 0° Abs. 

 Under these pressures there is a very great change in properties. 

 The thermal expansion drops to less than one-fourth of what it is 

 ordinarily, and the compressibility drops to one-fifteenth; both 

 of these changes are very much greater than the changes of volume. 

 The fact that compressibility and thermal expansion drop off so 

 greatly for comparatively small changes of volume suggests that 

 these must be intimately connected in some way with the empty 

 spaces between the atoms. All of the organic liquids behave roughly 

 alike, so that an average of them furnishes us with a sort of ideal 

 liquid, analogous to the familiar '' perfect " gas. This ideal liquid 

 has one curious property; above a certain pressure the ordinary 

 behavior of thermal expansion reverses itself, and the liquid is more 

 expansible at the low rather than at the high temperatures. In 

 addition to the broad features, which are alike for all liquids, each 



