556 , REPORT— 1903. 



The conditions favouralile for the formation of hoar frost occur so abundantly 

 in nature that it is surprising that the forms of natural hoar frost have been until 

 recently unknown, lu 1892 one of us found the walls of an ice cave in Iceland 

 coated with some remarkably fine hoar frost crystal, of a shape hitherto unknown, 

 but since then found by us to be the typical and principal form of hoar frost 

 crystals, viz., hollow hexagonal pyramids. 



It was found that the most favourable conditions for the formation of large 

 crystals are not only moist air at a low temperature, but also a quite undis- 

 turbed state of the air. For this reason the finest crystals are formed in caves or, 

 generally speaking, in closed spaces. 



With regard to the forms of hoar frost, the most typical form, as already men- 

 tioned, is a hollow hexagonal pyramid. It is built up upon a small flat hexagonal 

 prism, which springs from a solid wall ; round its edges a hexagonal step is formed ; 

 round the outer edge of this, again, another larger hexagonal step, and so forth, 

 exactly analogous to the hollow salt hopper crystals of the cubic system. Like 

 the latter, these hollow hexagonal pyramids are the product of the struggle for 

 attraction of material from the surrounding atmosphere ; the outer edges, having 

 a wider area to attract material from, grow more than the central portions, which 

 remain uncompleted. We have thus ' skeleton crystals ' formed ; the centre re- 

 mains undeveloped, due to the want of material — to ' starvation.' The greater 

 possibilities of attraction are well exemplified by the additional crystal formations 

 at the outer angles of the hexagons, quite similar to the * hopper ' crystals of 

 NaCl. 



The crystalline forms are exclusively those of the hexagonal fiat-topped prism ; 

 never has a terminal pyramid nor a hemihedral shape been observed. 



A great variety of forms of skeleton crystals can be observed under favourable 

 conditions ; amongst others, helix-shaped hollow pyramids, analogous to the cubic 

 helices of bismuth ; also long solid or helix-shaped hexagonal prisms. The hollow 

 pyramids are built of steps of prismatic rings, invariably with a basal pinacoid 

 face. 



Often a crystal shows needle-like spikes arranged in decided right angles. This 

 gives the strongest impression of cubic or other rectangular crystals. On careful 

 examination, however, it will always be found that in such a case we have to deal 

 with the incompletely developed rectangular faces of the hexagonal prism, such 

 as we might expect in a skeleton crystal. 



A series of micro-photographs and sketches will illustrate this. 



The most favourable conditions for the formation of hoar frost crystals and the 

 best opportunities for studying them are found in the refrigerating chambers as 

 used extensively in Liverpool, and through tlie kindness of the large shipping firm 

 of Messrs. Nelson an excursion and demonstration will be held there. 



Department op Physics. 



1. Discussion on the Treatment of Irreversible Processes in Thermo- 

 dynamics} Oldened by 3. Swinburne, M.Inst. C.E. 



The following Papers were read : — 



2. Note on the Bate of Combustion and Exjilosive Pressure of Cordite. 



By J. E. Petavel. 



The research of which a preliminary account was given is being carried out in 

 the physical laboratories of the Owens College. 



' Mr. Swinburne's contribution appeared in Engineering, August and September 

 1903. 



