138 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



pletely explain the fact that the movements of glaciers are 

 more rapid in proportion as the temperature rises ; and the 

 theory of glacial movements seems to receive through his 

 experiments a new support. Sitzh. Physikal. - Medecin. 

 Gesells.^ Erlangen^ 1875, 72. 



PLASTICO-DYNAMICS. 



Within the last few years a new branch of mechanics has 

 been developed by Saint- Yenant, known as plastico-dynam- 

 ics, which occupies itself with the movements in the interior 

 of plastic solid bodies; researches on this subject were made 

 by Tresca, and published in 1864 in a work on the flow of solid 

 bodies, and in a work published nearly at the same time by 

 Saint- Yenant on the torsion of prisms of various shapes. 



Subsequently, in 1870, Tresca published a work in which 

 he endeavored to explain the phenomena under considera- 

 tion by tracing out the movements of the molecules of the 

 bodies in question as they moved more or less slowly under 

 the influence of the pressures and resistances to which they 

 were subjected. His principle of the conservation of vol- 

 umes has been developed by Saint -Yenant, who has shown, 

 however, that it is not entirely adequate to explain the ob- 

 served phenomena, or to give satisfactorily the actual values 

 of the strain in the interior of solid bodies undersfoino^ de- 

 formation in consequence of external pressures. Both Tre- 

 sca and Saint -Yenant agree that the fundamental principle 

 of the new science amounts to this, that at every point in 

 the interior of a plastic body which is being deformed the 

 greatest tangential component of the pressure remains equal 

 to a siDCcific constant peculiar to each substance ; and from 

 this principle alone Saint -Yenant has recently established 

 certain differential equations of the plastic movements, an 

 integration of which would go far toward a complete solu- 

 tion of the problem. Failing in an analytical solution, how- 

 ever, he suggests that experiments must be resorted to, and 

 defines with some precision the nature of the desired obser- 

 vations. 6 B, LXXXL, 115. 



AN EXPERIMENT IN INSTANTANEOUS CRYSTALLIZATION. 



Peligot has recently described in La Nature the following 

 interesting experiment : Dissolve one hundred and fifty parts. 



