26G Sir James Dewar [Jan. 16, 



A great number of bodies were studied. Those solid at ordinary- 

 temperature were usually ground and polished to the requisite 

 thickness. If not obtainable in pieces of sufficient size, the powdered 

 or crystalline material was compressed hydraulieally into suitable 

 plates, sometimes with the addition of a trace of solvent. Fusible 

 substances were usually cast. Mica laminae were stripped in the 

 usual way ; while glass fragments of sufficient size (2| cm. disc) but 

 thinner than micro-cover glasses were selected from large thin bulbs. 

 The majority of observations were, however, made on materials 

 liquid at the ordinary temperature, although solid when cooled in 

 liquid air ; and various devices became necessary to get suitable 

 plates of these for immersion above the cell. Several could, by 

 careful partial cooling, be rendered sufficiently viscous to give a 

 coherent disc when poured on a quartz plate supported above a 

 liquid air surface. By further gradational cooling the viscous disc 

 could very often be solidified to a clear glass usually quite readily 

 separable from the quartz plate. The large number of substances 

 which g;ive clear glasses when properly cooled to liquid air temperature 

 is very extraordinary, a crystalline disc being almost exceptional. 



Another simple method was to use a tambourine of a rubber 

 membrane stretched on a light metal ring (2| to 3 cm. diameter). 

 This could be first carefully cooled, and the measured quantity of 

 liquid (also previously cooled as low as was practicable) poured in and 

 finally solidified by immersion. In some cases the rubber membrane 

 could' be stripped off under liquid air, leaving the thin solidified 

 plate : but this is not necessary if the small absorption of the 

 membrane is previously measured, and is very often not practicable, 

 as, e.g.. when cracks are formed during the cooling. Previous cooling 

 of the membrane and liquid separately allows reactive or very solvent 

 liquids to be readily handled without attack on the membrane or 

 its support ; but in many cases the tambourine was carefully cooled 

 with the liquid already in. The tambourine was supported by a 

 vertical wire bent to a springy horizontal ring, clamped above a cup 

 of liquid air, which was steadily screwed up to cool the membrane. 

 A deep quartz cup floating on the liquid air surface (but without 

 much lateral freedom) was an advantage in some cases where more 

 controlled and slower cooling was found necessary to give the best 

 results. 



Thus the ordinary condensable gases, such as ammonia, sulphurous 

 acid, hydrochloric acid, etc., could usually be dripped from the end 

 of a delivery tube just above the membrane tambourine, supported 

 at the appropriate height above the liquid air. After the liquid had 

 been solidified by further cooling, weighing of the tambourine on a 

 scale pan in an open vessel immersed in liquid air presented no 

 serious difficulties." The thickness of the condensed material was 



Proc. Boy. Soc, A. lxxxix. p. 158. 



