CATALOGUE. — METEOROLOGV, DEW, 



474 



but nearest to the light of the sky. But probably a differ- 

 ence of temperature was concerned. 



Hube iiber die ausdiinstung. 211. 



On honey dew. See the Author^ quoted by 



Lichtenberg in Eixleben.§. 730. 

 Prieuron dew. Joiirn. Polyt. II. vi. 409. Ann. 



Ch. XXV1II.317. Nich. IV. 86. 

 Hassenfratz on the evening and morningdew. 



Jouru. de I'Ecole Poiytechn. Ph. M. VII. 



114. 

 *B, Prevost on dew. Ann. Ch. XLIV. 75. 



Jomn. K. I., I. Nich. 8. III. 290. Gilb. 



XV. 485. 



Most of the facts may perhaps be explained by means of 

 Mr. Leslie's discoveries. 



Pallon on rain and dew. Manch. M. V. 



Makes the dew falling on grass about 5 inches annually, 

 or somewhat less. 



Account of a Mtmoir on Dew. By Benedict I'revost. 

 ' Ahridgtd from the Annalet de Chimie. No. 130. 

 Journ. R. I. I. 292. 



It is well I;nown that dew is often deposited on glass, 

 when metals in its neighbourhood remain dry ; Mr. Prevost 

 has however discovered some new and curious facts relative 

 to this deposition. When thin plates of metal are fixed on 

 pieces of glass, it sometimes happens that they are as much 

 covered with dew as the glass itself: but more frequently 

 they remain dry ; and in this case they are also surrounded 

 by a dry zone. But when the other side of the glass is ex- 

 posed to dew, the part which is opposite to the metal re- 

 mains perfectly dry. If the metal be again covered with ' 

 glass, it will lose its effect in preventing the deposition. 



These experiments may be very conveniently made on 

 the glass of a window, when moisture is attaching itself to 

 cither of its surfaces ; Mr. Prevost remarks that it often 

 happens that dew is deposited externally, even when the 

 air within is warmer than without. A plate of metal fixed 

 internally on a window receives a larger quantity of moisture 

 than the glass, while the space opposite to an external plate 

 remains dry : and if the humidity is deposited from without, 

 the place opposite the -internal plate is also more moistened, 

 while the external plate remains dry: and both these cir- 

 cumstances may happen at once with the same result. A 

 small plate fixed externally, opposite to the middle of the ' 

 internal plate, protects this part of the plate from receiving 

 moisture, and a smaller piece of glass, fixed on the external 



plate, produces again a cential spot^ of moisture on the in- 

 ternal one: and the same changes may be continued for a 

 number of alternations, until the whole thickness becomes 

 more than half an inch. Gilt paper, with its metallic sur- 

 face exposed, acts as a metal, but when the paper only is 

 exposed, it has no effect. When a plate of metal, on which 

 moisture would have been deposited, is fixed at a small dis- 

 tance from the glass, the moisture is transferred to the sur- 

 face of the glass immediately under it, without affecting 

 the metal : if this plate is varnished on the surface remoje 

 from the glass, the effect remains, but if on the side next 

 the glass, it is destroyed. The oxidation of metals renders 

 them also unfit for the experiment. When glasses partly 

 filled with mercury, or even with water, are exposed to the 

 dew, it is dei)Osited only on the parts which are above the 

 surface of the fluid. But in all cases when the humidity is 

 too copious, the results are confused. 

 ■ In order to reduce these facts to some general laws, Mr. 

 Prevost observes, that when the metal is placed on the 

 warmer side of the glass, the humidity is deposited more 

 copiously either on itself or on cither surface of the glass in 

 its neighbourhood : but that, when it is on the colder side, 

 it neither receives humidity nor permits its deposition on the 

 glass : that a coat of glass, or varnish, destroys the efficacy 

 of the metal, but that an additional plate of metal restores it. 

 Mr. Prevost was at first disposed to attribute these pheno- 

 mena to the effects of electricity; but he thinks it possible 

 to explain them all by the action of heat only : for this pur- 

 pose he assumes, first, that glass'attracts humidity the more 

 powerfully as its temperature is lower ; secondly, that me- 

 tals attract it but very little ; thirdly, that iglass exerts this 

 attraction notwithstanding the interposition of other bodies • 

 and fourthly, that metals give to glass, placed in their 

 neiglibourhood, the power of being heated by warm air, ani 

 being cooled by cold air, with greater rapidity ; hence that the 

 temperature of the glass approaches more nearly to that of 

 the air on the side opposite to the metal, and attracts the hu- 

 midity accordingly more or less, either to its own surface, 

 or to that of the metal. We should indeed have expected a 

 contrary cffi;ct ; that the metal would rather have tended to 

 communicate to the glass the temperature of the air on its 

 own side : but granting that the assumptions of Mr."Prevost 

 serve to generalise the facts with accuracy, their temporary 

 utility isas great as if they were fundamentally probable. Y. 



Kuin in general. 



Labile on rain water. A. P. 1703. 5(5. 

 Wargentin. Schw.Abh. XXV.3. 

 Leche. Sdiw. Abh. XXV. 16. 



