181 5.] Scientific Intelligence. 3'J3 



filled with coal gas, and the pressure by which it is made to sink in 

 the water as the gas is expended be taken off, I can easily conceive 

 that after an interval of a day or two (according to the size of the 

 lack), five-sixths of the gas may make i^s escape, and be replaced 

 by as much common air ; for as coal gas is mucii lighter than 

 common air, it will always make its escape wlicn it can. I am 

 persuaded that the explosion at Birmingham, and some others which 

 I have heard of, were produced in this manner. But as nothing is 

 easier than to ascertain whether the reservoir be air-tight, I conceive 

 that with common precaution gas lights may be used with as much 

 safety as any other light whate\tr. 



As to the coal that answers best, it is undoubtedly that variety which 

 contains the greatest quanthy o\ bitumen. Newcastle and Wigan 

 coal will probably answer better than any other coal in the ii;land, ex- 

 cept some of the Fife coal, which is absolutely of the same quality. 



With respect to the degree of pressure requisite to make it issue 

 with sufficient velocity from the mouth of the pipes, I do not know 

 that any accurate experiments have been made to determine it, nor 

 do I believe that much nicety is necessary. Some Gentlemen have 

 expressed doubts on the subject, because Mr. Wilkinson was not 

 able to force an efficacious draught of air through a pipe a few 

 hundred feet in length ; but tlie cases are by no means parallel. 

 Mr. Wilkinson required a current of considerable velocity; but no 

 such velocity is requisite for the coal gas. Besides, the water 

 generated by the combustion of the coal gas at the extremity of the 

 tube must occasion a diminution of pressure, which will serve to 

 regulate the issue of the coal gas from the reservoir. 



III. Toads found in Rocks arid Trees. 



A correspondent at Bristol, I. B. I., has proposed some queries 

 relative to the many stories in circulation respecting toads found in 

 the middle of solid rocks. There can be no doubt that toads have 

 been frequently found in such positions, though in no one case that 

 I have seen has it been ascertained that the animal was completely 

 excluded from the external air. I have myself conversed with 

 workmen who had found toads both in coal-pits and in quarries j 

 but they were never able to bring decisive proof that the animal was 

 completely surrounded by the coal or the stone, it is generally 

 observed that when a toad is found in this position the creature dies 

 very soon after being brought out of its lurking hole. This seems 

 to me a proof that the animal, if not entirely excluded from the 

 air, must have been at least nearly so; for the sudden death can 

 scarcely be ascribed to any thing else than the change of situation. 



IV. On the Focal Powers of (lie Ei/e. By Dan Pring, Surgeon. 



It has been presumed from tlie supposed resemliUnce of the eve 

 to some optical contrivance", that it is necessary the ft)riner should 

 possess a varial;Ie focal power, in order to account for the caj)ability 

 of viewing objects at various distances. But this supposition appears 



a 



