PROGRESS IN NINETEENTH CENTURY 33 



Crystallography 



Theoretical crystallography, approached by Steno (1669), but 

 formally founded by Haiiy (1781, Traiti, 1801), has limited its 

 development during the century to systematic classifications of 

 form. Thus the thirty- two type sets of Hessel (1830) and of Bravais 

 (1850) have expanded into the more extensive point series involving 

 230 types due to Jordan (1868), Sohncke (1876), Federow (1890), 

 and Schoenfliess (1891). Physical theories of crystalline form have 

 scarcely been unfolded. 



Capillarity 



Capillarity antedated the century in little more than the provi- 

 sional, though brilliant, treatment due to Clairaut (1743). The 

 theory arose in almost its present state of perfection in the great 

 memoir of Laplace (1805), one of the most beautiful examples of 

 the Newton-Boscovichian (1758) molecular dynamics. Capillary 

 pressure was here shown to vary with the principal radii of curva- 

 ture of the exposed surface, in an equation involving two constants, 

 one dependent on the liquid only, the other doubly specific for the 

 bodies in contact. Integrations for special conditions include the 

 cases of tubes, plates, drops, contact angle, and similar instances. 

 Gauss (1829), dissatisfied with Laplace's method, virtually repro- 

 duced the whole theory from a new basis, avoiding molecular forces 

 in favor of Lagrangian displacements, while Poisson (1831) obtained 

 Laplace's equations by actually accentuating the molecular hypo- 

 thesis; but his demonstration has since been discredited. Young 

 in 1805 explained capillary phenomena by postulating a constant 

 surface tension, a method which has since been popularized by Max- 

 well (Heat, 1872). 



With these magnificent theories propounded for guidance at 

 the very threshold of the century, one is prepared to anticipate the 

 wealth of experimental and detailed theoretical research which 

 has been devoted to capillarity. Among these the fascinating mono- 

 graph of Plateau (1873), in which the consequences of theory are 

 tested by the behavior both of liquid lamellse and by suspended 

 masses, Savart's (1833), and particularly Rayleigh's, researches 

 with jets (1879-83), Kelvin's ripples (1871), may be cited as typ- 

 ical. Of peculiar importance, quite apart from its meteorological 

 bearing, is Kelvin's deduction (1870) of the interdependence of sur- 

 face tension and vapor pressure when varying with the curvature of 

 a droplet. 



