204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



pressure of 100,000 atmospheres is generated is, on tlie inside, only 

 1.6 mm. in diameter and 8.0 mm. long. In this apparatus, volume 

 changes can be measured with fair precision, and the volume changes 

 of a large number of substances have been determined. Still higher 

 pressures can be reached, but, at this stage, on a scale so small that 

 little research of scientific value has been done. The principle by 

 which these higher pressures may be reached is that used in making 

 hardness measurements by pressing a Brinell ball or a diamond 

 cone into the material being tested. Stresses very much higher than 

 normal may be supported over small areas if the surrounding mate- 

 rial is unstressed and so can support the highly stressed region. 

 With an arrangement of this sort, in which a short carboloy cone 

 is pressed against a flat carboloy block, the whole combination being 

 mounted within a chamber at 30,000 atmospheres so that there is 

 additional support by hydrostatic pressure, pressures in excess of 

 400,000 atmospheres have been realized at the point of contact of the 

 cone. However, not much use can be made of these very high pres- 

 sures, except to achieve the negative result of showing that certain 

 transformations that might perhaps be expected are not in fact 

 produced. In particular, graphite is not transformed into diamond 

 by such a pressure at room temperature, although it is the thermo- 

 dynamically stable form. 



All the pressures mentioned so far have been static pressures. 

 Dynamically, as in the explosion of shaped charges, it is possible to 

 reach very much higher pressures, measured in millions of atmos- 

 pheres. This is doubtless the ultimate method of getting high pres- 

 sures (except by the use of atomic bombs), and a beginning is now 

 being made. The difficulties, however, to say nothing of the expense, 

 are immense, and progress will probably be slow. The problem of 

 measuring the pressures and temperatures reached by such methods 

 is itself an exceedingly formidable one, and for the present the only 

 method seems to be to extrapolate results obtained by static methods 

 in lower ranges. Thus the results that can be obtained by the static 

 methods outlined above will probably continue to have their useful- 

 ness for some considerable time. 



Having now discussed the various methods of achieving high pres- 

 sures we may turn to a consideration of some of the effects produced. 

 The simplest of these effects, although by no means the simplest to 

 measure, is the diminution of volume which all substances suffer 

 under pressure. A general feature of the volume changes produced 

 by hydrostatic pressure is that they are reversible, so that when pres- 

 sure is released the volume recovers its original value. In other 

 words, there is no elastic limit or fracture point. This is strikingly 

 different from what happens when change of shape is produced by 



