322 REVIEWS 



3. Pressure alone is not so efficient as when accompanied by an 

 agitation of the particles which breaks their cohesion and aids diffusion. 

 This explains cold welding, etc. 



4. The three states of matter are only extreme degrees of a single 

 ne and each has a critical pressure and temperature. Solids as well 



as gases and liquids have faster and slower moving molecules and this 

 variation is the most extreme where the free path is the greatest, viz., 

 at the surface, and therefore solid bodies in contact will weld at their 

 surfaces when below the melting point, due to the interaction of the 

 ■"liquid molecules." 



5. The molecular mobility of a solid body is a function of the pro- 

 portion of the rapidly moving portion (liquid molecules) to the whole 

 number of molecules. 



6. Crystalline bodies have a nearly uniform rate of molecular 

 vibration, and therefore do not "solder" at a temperature much below 

 melting, while amorphous or partially crystalline bodies with hetero- 

 geneous molecular motions weld easily. 



Finally, in the contributions of the last few years 1 he has described 

 some of his most remarkable experiments on this subject which con- 

 firm all his former conclusions. He kept perfectly dry powdered chalk 

 under a pressure of from 6000 to 7000 atmospheres for seventeen 

 years and three months. This same pressure acting through a short 

 time only makes the chalk about as hard as ordinary writing crayon, but 

 after this long time it was found as hard as marble. The fracture was 

 ■conchoidal and the microscope showed it was crystalline. The steel 

 screw which held the chalk under pressure had completely united to 

 the cylinder, so that the cylinder had to be cut. The chalk was of a yel- 

 low ochre color to the depth of ij4 mm , showing that the iron molecules 

 had diffused i*4 mm into the chalk in about seventeen years. 



This diffusion was at the ordinary temperature, but he has shown 

 that at higher temperatures the velocity is much greater and " not only 

 tends to complete the homogeneity of the solid solution, but also to 

 cause an orientation of its molecules." These observations he applies 

 as follows: "Les faits que j'ai observe contribueront peut-etre a Jeter 



^ull. de 1'Academie, etc., 3 me serie, 1895. De l'influence du temps sur l'agglu- 

 tination de la craie comprimee. 



Sur les modifications physiques que subissent certains sulfures sous l'influence de 

 la temperature. Bull, de l'Academie, 3 me serie, 1895, p. 311. 



Zeit. f. Phys. Chem. 



