Force of Vapour. 181 



of swimming, but are furnished with pincers and feet, almost 

 solely adapted to crawling, so that they are now under the ne- 

 cessity of confining their excursions in pursuit of prey, within 

 more narrow limits. This curious piece of economy, explains 

 what has ever appeared paradoxical to naturalists, viz. the an- 

 nual peregrinations of the land crabs to the sea side ; which, 

 although acknowledged to be true, by several competent ob- 

 servers, could never before be satisfactorily accounted for." 



FORCE OF VAPOUR. 



Sir, — By publishing the following extract from Professor Silli- 

 man's Elements of Chemistry, and the subjoined notes, you will 

 oblige A Subscriber. 



It is stated in Professor Sillman's Elements of Chemistry, pp, 

 87, 88, and 105, Vol. I. that " The force of vapour formed at the 

 boiling pohit, is the same in all fluids, and just equals the pres- 

 sure of the atmosphere, that is, thirty inches of mercury : this 

 law appears to be fully established. 



" It was also stated, in the same connexion, that the force of 

 vapour is the same for every fluid, at an equal number of degrees 

 above and below its boiling point. Both these laws were origin- 

 ally given on the authority of Mr. Dalton, which was sustained 

 by that of other eminent philosophers. Dr. Turner, in the second 

 edition of his chemistry, (third Amer. p. 61,) says, 'The force of 

 vapour from all liquids is the same, at equal distances above or 

 below the several temperatures at which they boil in the open 

 air. Thus, steam at 200° F., has the same elasticity as the va- 

 pour of ether at 85°; the boiling point of the former being 212°, 

 and of the latter, 97° : Biot and Amede Berthollet (Biot Traite de 

 Ph. I. 282) have found that this law applies exactly to many 

 other Uquids ; but some experiments on the oil of turpentine and 

 petroleum, would lead to the conclusion that it was not universal.' 

 Dr. Thompson (on heat and electricity, p. 222,) states, that by 

 subsequent experiments, the second law of Mr. Dalton has not 

 been found to hold good, and that it has been relinquished by its 

 author. At the time when my statement of those law^s was writ- 

 ten and printed, neither the second English edition of Dr. Turner 

 had appeared, nor the work of Dr. Thompson, nor the eleventh 



