242 STUDIES OF NATUHE. 



once, you fee, in a fhort time, ice of an ifich 

 in thicknefs cover the outfide of the bottle. A 

 block of ice diffufes cold all over the furrounding 

 atmofphere. Darknefs, neverthelefs, which is a 

 privation of lights ditfufes no obfcurity over fur- 

 rounding light. If you open, in a day of Sum- 

 mer, a grotto at once dark and cool, the furround- 

 ing light will not be in the leaft impaired by the 

 darknefs which it contained -, but the heat of the 

 adjacent air will be perceptibly diminiflied by the 

 cold air which iffbes from it. I am aware of the 

 reply j it will be faid, if there is no perceptible 

 obfcuration in the firfl: cafe, it is owing to the ex- 

 treme rapidity of light, which replaces the dark- 

 nefs ; but this would be increafing the difficulty, 

 inftead of removing it, by fuppofmg that darknefs, 

 too, has pofitive effects, which we have not time 

 now to animadvert upon. 



It is, however, on fuch pretended fundamental 

 principles, that moft of our fyftems of Phyfics are 

 reared. If vve are in an error, or in a ftate of ig- 

 norance, at the point of departure, it cannot be 

 long before we go aftray on the road ; and it is 

 really incredible with what facilitj', after having 

 laid down our principles fo flightly, we repay our- 

 felves in confequences, in vague terms, and in 

 contradidory ideas. 



I have 



