in the drying of a Fluid with Particles in Suspension. 225 



mixed with material in suspension (e.g., calcium sulphate and ver- 

 milion), a mass of crystalline grains, separated by clear, narrow 

 interspaces, occupy a definitely crystal-shaped area. The neigh- 

 bouring grains are related in form, the spaces between are bent or 

 curved, the whole resembling micropegmatitic or pegmatitic struc- 

 ture (fig. 5.) 



In the formation of chiastolite and other secondary minerals, the 

 matrix is often partly included, but in these experiments the more fluid 

 medium apparently separated from the vermilion, thrust it to the edge 

 of an initial, crystal -shaped area, and within that formed crystalline 

 grains separated by interspaces. The grains generally depolarise 

 uniformly, but occasionally are built up of clustered prisms, and some- 

 times even of slender, tufted needles. Lehmann, in his classic paper, 

 describes the causes of irregular, or interrupted growth as due 

 mainly to the viscosity of the medium or the presence of foreign 

 substance .* He points to spherulitic and dendritic forms as results. 

 The micropegmatite seems to be one more possible development. f 

 This would agree with the hypothesis advanced by Professor Bonney, 

 that it results in igneous rocks, when the magma is kept at a some- 

 what persistent, but not too high, temperature ; so that the material 

 probably would be in a very viscous or partly solidified condition. J 



While this paper was in progress, I received from Professor 

 Bonney the following interesting note (drawn up by him some years 

 ago), the more valuable as made upon one of nature's experiments. 

 The observation adds one more suggestion as to the possible forma- 

 tion of "pseud-organic" structures, that they might originate from 

 a mixture of mechanical sediment and of a crystallising salt, and 

 the forms in the mud might remain even if the salt were afterwards 

 dissolved. 



The note is as follows : " In walking along the pavements during 

 the late frost, before the sun or the feet of the public had produced 

 an effect, I was often struck with the forms of the ice crystals. v The 

 pavements were dirty ; much fine mud, brought from the roads on 

 the boots of pedestrians, had been pretty evenly distributed in a 

 thin film. During the night this had been arranged in rod-like 

 crystals, often 3 or 4 inches long. These formed groups, spreading 

 like the sticks of a partly opened fan. They were sometimes 



* ' Zeits. fur Kryst.,' 1877, vol. 1, p. 453. 



f Micropegmatite is closely related to the other forms. 



" On a Contact Structure in the Syenite of Bradgate Park," ' Quart. Journ. 

 Geol. Soc.,' 1891, vol. 47, p. 107. 



Of this note, written in December, 1892, an abstract was sent to ' Nature ' 

 (December 15, 1892) by the author in corroboration of a letter, which had 

 appeared the previous week, from Professor Meldola, calling attention to the 

 same phenomenon. Several letters (from Dr. J. H. Gladstone and others) on the 

 same subject were printed at the same time. ' Nature,' vol. 47, pp. 125, 162. 



R 2 



