floa On CapV.Iary ASIion. 



be aflifted by the concurrence of other agents tliat tend to 

 difimite the Iblid. In general, the action of chemical affini- 

 ties admits of compariion with the application of mechanical 

 force. There are fome obvious facets which at once illuftrate 

 and confirm this principle. For example, carbonic gas, com- 

 bined with water, enters into the compofition of marble; but 

 neither of thofe fugitive fubRances, however concealed or 

 difguiied, yet abandons its fpecilic character. On the ap- 

 plication of intenfe heat their elaflicity becomes developed 

 with irrefiftible force, and they make their efcapc, leaving 

 the Itony bafis in the drv cauftic Hale of quicklime. I fliall 

 mention one other faft which exhibits the play of chemical 

 affinities. Charcoal, it is well known, has the property of 

 imbibing air ; and 1 have proved from experiment that this 

 iniprifoncd air exifts in a comprclfed or condenied ftatc. But 

 water, being applied to the charcoal, will be immediately 

 abforbed by fuperior attraftion, and vvill diflodge the air, 

 which now recovers its ufual expanfion. 



Sulphuric acid, potafli, and in general all the deliquefcent 

 falls, have the power of altrafting moifturc from the atmo- 

 fphere. They exert a real chemical acition which overcomes 

 the adhefion of humidity to the air. But I have remarked 

 the fame property to obtain more or lefs in a great varietv of 

 fubftances; in paper, linen, flannel, wood; nay, in (tones 

 and earths. The water, thus abli rafted from the atmofphere, 

 penetrates into their fubftance and difuppears. The furface 

 of glafs fhows, in a limited degree, a fimilar power. I have 

 obferved that, on the approach of evening, bits of glafs be- 

 came covered over with minute globules long before the fur- 

 rounding air was abfolulelv damp. About the temperature 

 of freezing I have noticed this dew to appear on the furface 

 of the glais, when mv hygrometer ftill indicated a drvnefs of 

 five degrees. But in higher temperatures the effeCt was even 

 more lirikmg; and I have feen the fides of a tumbler covered 

 with dew when the ambient air polTeffed 15 degrees of dry- 

 refs. In thofe cafes, however, the humidity is merely de- 

 tached from the air by the aftion of the glafs, and it remains 

 adhering to the furface without being abforbed into the vi- 

 treous mafs. The etVcCt cannot, with any appearance of pro- 

 bability, be imputed to a fuppofed alkaline efflorefcence ; fur 

 it was obferved with fine flint glafs, and the dew coUeftcd 

 was always pcrfe<ftly taltelefs. When polifhed metals were 

 cxpofcd, thev never received any dew till the atmofphere was 

 abfolutely damp. They feem therefore to be entirely pnflive. 



This fing^ular property of glafs is certainly a modification 



of 



