126 



NA TURE 



[December 8, 1892 



Road, and other thoroughfares having an east and west direc- 

 tion the paving flags were all covered with a striking, vegetable- 

 like pattern which might be most appropriately described as an 

 arborescent tracery. The pattern was not formed of the usual 

 small and delicate frost figures such as we are familiar with on 

 window panes, but was made up of large and boklly-fronded 

 designs such as shown in the sketch, which 1 hurriedly made on 

 the spot : — 



The "fronds" were from one to two feet in length, and often 

 most gracefully curved. A keen wind was blowing at the time 

 from 'a few degrees north of west and the flags had evidently 

 been coated with a thin layer of mud from the previous night's 

 rain. I attribute the pattern to the rapid freezing and evapora- 

 tion of the water in this surface layer of mud which was going 

 on during the morning. I only noticed the tracery along east 

 and west thoroughfares ; in sheltered streets not swept by the 

 cold wind no design was visible. The phenomenon may be 

 known well enough to others, but by many, like myself, it may 

 have hitherto been passed over unnoticed. My chief object in 

 sending this description is to call attention to the very vegetable- 

 like appearance of the pattern. If allowed to dry in a calm 

 atmosphere and then buried under a fine alluvial or other de- 

 posit a record would be preserved which the future geologist 

 might at first sight be tempted to read as " vegetable remains." 

 I have seen very similar tracery in the London clay about 

 Clacton-on-Sea and elsewhere. R. Meldola. 



cause oi certain structural phenomena observed in them from 

 lime to time. It was with those ideas present to my mind that 

 during recent winters I have made an examination of the acicu- 

 lar ice-forms referred to, which struck me as made up of unusually 

 clear and transparent ice. On taking my microscope out of 

 tloors, fitted with a polarizing apparatus, when the temperature 

 was a few degrees below freezing, with a thick overcoat on to 

 prevent the heat of one's body froui affecting the ice-needles, I 

 ibund that, on taking them from the giound and placing them at 

 once on the stage between crossed "Nicols," they appeared to 

 be completely isotropic^ as they had no reaction on polarized light. 

 I have concluded, therefore, that these ice-needles are strictly 

 analogous (physically) to the prismatic crystallites of sul- 

 phur; and they resemble precisely the micioscopic lathe-shaped 

 forms, into which I have seen a perfectly clear minute plate of 

 sulphur-glass break up in the first stage of devitrification. The 

 explanation suggested by Mr. Woodd Smith, that they may 

 have been formed by a slow growth of ice at their base, the 

 molecular movement of water in the soil keeping up the supply 

 so long as refrigeration continued, has seemed to me the most 

 natural one ; their isotropic molecular structure is no doubt due 

 to the rapidity of freezing owin^ 1 1 a sudden fall of temperature 

 at the spot. A. Irving. 



Wellington College, Berks, Noveaiber 27. 



Ice Crystallites. 



The interesting facts recorded by your correspondent 

 C. M. Irvine on p. 31 recall some unrecorded observa- 

 tions of my own. On several occasions during recent winters 

 I have observed these crystal lographic forms of ice on a 

 gravel walk by the side of my lawn, in places where, owing to 

 taulty gradients, the water does not completely drain away at the 

 surface, and the ground just below the surface is in consequence 

 more saturated with water than at other spots. Theacicular ice- 

 forms have appeared in bundles standing up between the pebbles 

 and capped by earthy material, just as described by Mr. Irvine, 

 and in previous communications to Nature by Mr. B. Woodd 

 Smith (see his letter on p. 79). The nature of the soil 

 agrees with that described by these two observers, so far as 

 permeability to water is concerned ; and I think they appeared 

 on the occurrence of clear frosty weather after a thaw and melt- 

 ing of previous snow. My observations, however, extended 

 further than theirs appear to have done. 1 was'at the time pur- 

 suing the study of the glassy acicular crystallites of sulphur 

 (which are erroneously described as "crystals" in most text- 

 books on chemistry). These, on examination with polarized 

 light (as I have described elsewhere) are found to be destitute of 

 any crystalline internal structure (in fact truly vitreous or isolroi)ic 

 masses in spite of their crystallographic outlines) ; such structure 

 developing, as devitrification proceeds, by crystallization in the 

 orthorhombic system, to which the outlines of the crystallites do 

 not conform. 



In Nature (vol. xxxvii. p. 104) is a letter from myself, 

 recording some observations on the vitreosity of ice, as exhibited 

 under certain suitable conditions by hailstones, and referring to 

 a previous letter {Ibid. vol. xxxvi. p. 77), wherein the vitrifica- 

 tion and devitrification of water was suggested as the possible 



The Fo/wr<'//(r as Alleged Examples of Variation "almost 

 Unique among Animals." 



It is barren work for the parties in a controversy merely to 

 deny each other's statements without adducing further evidence. 

 Mr. Bateson first stated that var. mystacea did not mimic 

 Bombus nmscorum. I replied that it did, and the statement in 

 my letter in no way depended on the case at the Royal College 

 of Surgeons, but on a careful comparison of the insects in the 

 Oxford Museum. It is useless for me to repeat that I regard it 

 as an example of mimicry, not indeed equal to that afforded by ' 



the same fly and Bombus horto}-um, but far better than many 

 others which are generally believed to be ins'ances of this 

 principle (such as the resemblance of Clytus arictis, or even the 

 resemblance — admitted by Mr. Bateson in his first letter — of 

 Volucclla inanis, to a wasp). I therefore propose to furnish 

 the Editor of Nature with photographs of the Volucellce and 

 I humble-bees for reproduction, so that readers can judge of the 

 matter for themselves. I will do my best to obtain a negative 

 which shows the coloured bands. 



Although I believed that the two London Museums supported 

 my view, it will be obvious to any one who reads the letter that 

 I did not rely on such support, but on my own comparison of 

 the insects. 



Mr. Bateson has offered no further evidence in support of his 

 remarkable assertion that the variation of the Volucellce is 

 unique. I am not surprised that he should pass over this part 

 of my letter, for I felt sure that there was no further evidence to 

 offer. It will be remembered that this evidence was contained 

 in the "brief statement of facts" given in his first letter, and 

 is practically summed up in the sentence " This fly exhibits the 

 rare condition of existing in two distinct forms in both sexes." 

 In assuming this rarity to be so excessive that the words " al- 

 most unique" may be applied to it, and in evidently considering 

 that we must proceed as far as the peach and nectarine in order 

 to find a parallel, Mr. Bateson exhibits a want of acquaintance 

 with the facts of variation which is very surprising in one who is 

 believed to have spent some years in their study. For there is i 



no essential biological difference between this variation and 

 many others, examples of which I gave in my last letter, and 

 which could easily be multiplied. In fact, many a " showcase " 

 would have corrected such a mistake. Compared with the 

 magnitude of this erroneous statement in Mr. Bateson's first 

 letter, the details under discussion assume very small propor- 

 tions. In considering that "no speculation is needed to enhance 

 the exceptionally interesting facts of the variation and the re- 

 semblances of the Volucella;," ii would appear that Mr. Bateson 

 seeks to replace that most invaluable servant of science, specula- 

 tion, by far-reaching and unsupported assertion. 



In his last letter Mr. Bateson says " it is admitted that in 

 making this statement Mr. Poulton relied not on original 

 authorities, but on the general impression of others." So far 

 from this being the case I stated my belief that the impression 

 is prevalent among those who are original authorities on the 

 Hynienoptera and their parasites, and I also showed that nothing 



NO. 1206, VOL.^47] 



