There are very few popular beliefs of any duration, however extravagant or incredible, that cannot be traced to some foundation in truth, however much distorted by ignorance, superstition or folly; and we have a remarkable instance of this in the celebrated Upas, or Poison Tree of Java, whose shade was believed to extinguish life in the unhappy beings who sought refuge under it. It was stated to be in a valley in the interior of Java, but it was surrounded with so many terrors, that its exact locality was not likely to be well defined or understood; and in this uncertainty originated the fables which have so long been before the public. I am indebted to Sir Charles Forbes, for a copy of a letter addressed to the late W. Taylor Money, Esq., Consul-General at Venice, from a gentleman who visited the Guwo-Upas, or Poisoned Valley, near Batur, in Java, on the 4th of July, 1830. I understand that the letter has appeared in print, but I have not seen it; and I deem it necessary to incorporate it in the present paper, to facilitate the comparisons and deductions I purpose making. A perusal of it will, I presume, afford satisfactory reasons to conclude, that in this deadly spot originated the belief in the Poison Tree, the mistake of the mephitic vapour escaping from vegetation, rather than from the soil, being natural and probable. The writer of the letter is a gentleman of the name of Loudon, an Englishman, but a landholder in Java, well known to Doctor Horsfield, and full reliance may be placed on the accuracy of his descriptions. He is disposed to question the resemblance between this Valley of Death and the Grotto del Cane, near to Naples; but I will endeavour to show that the difference is only in the physical features of the localities, and that the probability is, that the effects described originate in precisely similar causes. Dr. Horsfield informs me that he was at Batur in 1815 and 1816, and aware of the vicinity of the poisonous valley, but the natives refused to conduct him to it.