222 



ROBERT B. SOSMAN 



Waxes and oils were used for most of Benard's experiments, 

 because at his working temperatures of ioo° and lower the requisite 

 conditions as to viscosity and low volatility could best be obtained 

 with these materials. By suspending in them finely powdered sub- 

 stances such as graphite or lycopodium, Benard was able to show 

 visually and to photograph the cells produced, without the aid of 

 special optical devices. 



As is to be expected, the dimensions of the cells depend upon the 

 thickness of the liquid layer, the temperature difference between 

 top and bottom, and the viscosity and temperature of the liquid. 



Fig. 3. — Three examples of hexagonal cells formed in a melted wax, taken under 

 different conditions of temperature and thickness and before the final steady state of 

 circulation had been attained. 



In a given liquid at a given temperature, and at a constant tem- 

 perature difference, the ratio of diameter to height is found to be 

 constant. Other laws governing the form and size of the cells were 

 found by Benard, but it is unnecessary to discuss these in detail. 



Following Benard, Dauzere 1 in 1907 showed that crystallization 

 in salol and wax mixtures begins on the boundaries of the convec- 

 tion cells. A mixture of beeswax and stearin, on solidifying, sepa- 

 rates spontaneously into hexagonal prisms coinciding with the 

 original convection cells. In pure stearin, also, crystallization 

 begins at the corners of the cells. In every case the cells leave a 

 permanent record of their existence in the crystallized solid, 

 although in some cases the structure is quite invisible, and only 



1 C. Dauzere, Jour, physique, VI (1907), 

 franc, av. sci., 1908, pp. 289-96. 



>2~99; VII (1908), 930-34; Assn. 



