[38] Rev. Walter Gregor, _Notes on the Folk-lore of the North-East of Scotland_ (London, 1881), p. 184. As to the superstitions attaching to stone arrowheads and axeheads (celts), commonly known as "thunderbolts," in the British Islands, see W.W. Skeat, "Snakestones and Stone Thunderbolts," _Folklore_, xxiii. (1912) pp. 60 _sqq._; and as to such superstitions in general, see Chr. Blinkenberg, _The Thunderweapon in Religion and Folklore_ (Cambridge, 1911). [39] Pliny, _Naturalis Historia_, xxix. 52-54. [40] W. Borlase, _Antiquities, Historical and Monumental, of the County of Cornwall_ (London, 1769), pp. 142 _sq._; J. Brand, _Popular Antiquities of Great Britain_ (London, 1882-1883), i. 322; J.G. Dalyell, _Darker Superstitions of Scotland_ (Edinburgh, 1834), pp. 140 _sq._; Daniel Wilson, _The Archaeology and Prehistoric Annals of Scotland_ (Edinburgh, 1851), pp. 303 _sqq._; Lieut.-Col. Forbes Leslie, _The Early Races of Scotland and their Monuments_ (Edinburgh, 1866), i. 75 _sqq._; J.G. Campbell, _Witchcraft and Second Sight in the Highlands and Islands of Scotland_ (Glasgow, 1902), pp. 84-88; Marie Trevelyan, _Folk-lore and Folk-stories of Wales_ (London, 1909), pp. 170 _sq._; J.C. Davies, _Folk-lore of West and Mid-Wales_ (Aberystwyth, 1911), p. 76. Compare W.W. Skeat, "Snakestones and Stone Thunderbolts," _Folk-lore,_ xxiii. (1912) pp. 45 _sqq._ The superstition is described as follows by Edward Lhwyd in a letter quoted by W. Borlase (_op. cit._ p. 142): "In most parts of Wales, and throughout all Scotland, and in Cornwall, we find it a common opinion of the vulgar, that about Midsummer-Eve (though in the time they do not all agree) it is usual for snakes to meet in companies; and that, by joining heads together, and hissing, a kind of bubble is formed, which the rest, by continual hissing, blow on till it passes quite through the body, and then it immediately hardens, and resembles a glass-ring, which whoever finds (as some old women and children are persuaded) shall prosper in all his undertakings. The rings thus generated, are called _Gleineu Nadroeth_; in English, Snake-stones. They are small glass amulets, commonly about half as wide as our finger-rings, but much thicker, of a green colour usually, though sometimes blue, and waved with red and white." [41] Pliny, _Naturalis Historia_ xxiv. 12 and 68, xxv. 171. [42] Marcellus, _De medicamentis_, ed. G. Helmreich (Leipsic, 1889), preface, p. i.: "_Nec solum veteres medicinae artis auctores Latino dumtaxat sermone perscriptos ... lectione scrutatus sum, sed etiam ab agrestibus et plebeis remedia fortuita atque simplicia, quae experimentis probaverant didici_." As to Marcellus and his work, see Jacob Grimm, "Ueber Marcellus Burdigalensis," _Abhandlungen der koniglichen Akademie der Wissenschaft zu Berlin_, 1847, pp. 429-460; _id._, "Ueber die Marcellischen Formeln," _ibid._. 1855, pp. 50-68. [43] Marcellus, _De medicamentis_, i. 68. [44] Marcellus, _op. cit._ i. 76.