36 Hugh Low, _Sarawak_ (London, 1848), pp. 145 _sq._
37 Pliny, _Naturalis Historia_, xxviii. 33 _sq._
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 arrow-heads and axeheads (celts), commonly known as "thunderbolts," in the British Islands, see W. W. Skeat, "Snake-stones and Stone Thunderbolts," _Folk-lore_, 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."