S98 lletrospedive Criticim, 



test of experiment, on the 14th of December, 1828, 1 captured eighteen 

 spiders of this species, twelve of which were enclosed in glass phials closely 

 corked -, and the other six were put on an upright twig whose base was im- 

 mersed in water ; which twig was placed within an inch of the perpendicular 

 side of the vessel containing the liquid. Both the phials and the vessel 

 containing the twig were then carefully locked up in a book-case, where 

 they remained till the 4th of January, 1829. During this period, five only, 

 of the twelve spiders put into the phials, died ; the remaining seven,"with the 

 six placed on the twig, which were quite unable to effect their escape, not 

 only survived, but retained their wonted vigour and activity. They were 

 then all set at liberty but one, which accidentally remained on the twig till 

 the 23d of February, when it also was suffered to escape. So much for the 

 report of the speedy dissolution of these spiders in a state of confinement, 

 and for the assertion founded upon it, that their ascent in the atmosphere 

 is essential to their existence. Perhaps, the result of this experiment might 

 be influenced, in some degree, by the season of the year, as I readily admit 

 that spiders enclosed in glass phials do not always survive so long as those 

 did in the instance given, though the average term of their existence in cap- 

 tivity greatly exceeds the limits assigned to it by Mr. Murray ; and I have 

 repeatedly kept spiders on the twig alluded to above, for three weeks and a 

 month, without any very sensible change being produced either in their 

 ph3'sical powers or their external appearance. 



In treating upon the electrical character of the lines of this species of 

 spider, Mr. Murray observes {Experimental Researches^ p. 1 36.), that " when 

 a stick of excited sealing-wax is brought near the thread of suspension, it is 

 evidently repelled ; consequently the electricity of the thread is of a negative 

 character ; " and that " an excited glass tube brought near seemed to attract 

 the thread, and with it the aeronautic spider." Here, again, my experience 

 is opposed to that of Mr. Murray. Like other light substances, I find that 

 the lines and spiders are attracted both by excited sealing-ivax and glass 

 tubes ; and that when uninfluenced by the contact, or vicinity of excited 

 bodies, they appear to be neutral with regard to electricity, or rather, not to 

 he in a state of excitation themselves, being affected neither by the ap- 

 proximation of the Jinger, nor of metallic conductors. That an electrical 

 atmosphere will impart electricity to the lines of spiders floating in it, is 

 highly probable, but then it is plain that the ascent of the spiders cannot be 

 occasioned by this circumstance. 



As it is almost impossible that observations and experiments on the 

 ascent of spiders in the open air should be repeated under circumstances pre- 

 cisely similar to those under which they were originally made. Instead of 

 recapitulating and enforcing my former statements relative to this fact, I 

 beg to refer Mr. Murray to a work of great interest, which I have recently 

 perused for the first time ; I allude to the Tableaux de la Nature of M. 

 de Humboldt, the most scientific traveller of the present day, where he will 

 find (tome ii. p. 4. & 59 — G2.), that not only insects of various kinds, but 

 inanimate objects also, are sometimes carried to an astonishing elevation in 

 the atmosphere by the mechanical action of ascending currents of rarefied 

 air. 



There are several other points of minor importance respecting which I 

 entirely dissent in opinion from Mr. Murray, but I have not leisure to enter 

 into an exposition of them at present; I trust, however, that I have suf- 

 ficiently elucidated some of those particulars which it would be most ad- 

 visable for him to correct in the intended new edition of his Experimental 

 Researches. On them I recommend him to bestow a careful and impartial 

 consideration without delay, and my sincere wish is, that he may derive all 

 the advantage from them which they are so eminently calculated to afford 

 him. I am, Sir, &c. — John Blackwall. Crumpsall Hall, July 18. 1829. 



Influence of Fresh Water on Marine Animals. — It may be remarked that 

 Pr. Drummond's experiments on the Nereis caM'idea, as recorded in your 



