STATE OF VAPOUR SUBSISTING IN THE ATMOSPHKRf . 215 



Thefe variations of the rates of expanfibilify of moid air, afcribed to dif- 

 faturated at different temperatures, Schmidt very juftly attri- J e " n a c t %°ff"^ t 

 bates to the variation of the degrees of affinity or adherence temperatures. 

 of air and vapour to each other at different temperatures. At 

 32 Q Fahr. it is very ftrong, and alfo below that degree ; and 

 hence the ftrong folvent power of air, colder than the water 

 it acts upon, remarked by Richman ; but if both are equally 

 cold very little moifture will be taken up by the air, as already 

 mentioned ; and hence I have faid that air diflblves vapour 

 when this is in a nafcent ftate. But in heats above 167° or 

 170° air and vapour are difpoied to feparate. 



5. Hence we may deduce the impoflibility of difcovering a Hence there 



coefficient univerfaliy applicable to exprefs the rate of expan- ca . n be " od <L ter - 



_ . . n . . minate coeffi- 



uon of air in every ftate of moifture,, (as Tremley has well cient to exprefc 



noticed. See 2 Sauflure Voy. aux Alpes 4to). This mud *** ratc * 

 vary with the mean ftate of hygrometers above and below the 

 heights to be meafured ; and experiments of this kind have 

 not yet been made. De Luc's coefficient anfwers tolerably 

 well for very dry air, that is whofe faturability is greateft, Sir 

 George Schuckburg's for air much moifter,*and general Roy's 

 for air ftill more moift, that is whofe faturability is fmalleft. 

 Hence each fucceeds in certain cafes, and fails in others. The 

 dilatation or contraction, which air faturated with moifture at 

 any one given degree of temperature receives without the ad- 

 dition of any more moifture, at any higher or lower degree of 

 temperature, has not as yet been difcovered; for Schmidt, 

 who alone has attempted it, is juftly diffident of the correcl- 

 nefs of the table he has given of it, and in fact it is not 

 grounded on the indication of any known hygrometer, and 

 improperly fuppofes the L. degree to indicate the mean be- 

 twixt the loweft and faturation. Whereas the LXVth degree 

 on Sauffure's indicates that mean; and XCVIIL and not C. 

 indicates faturation. 



According to Mr. Watt (as ftated by De Luc, 3 Meteor o- Specific gravities 

 log : p. 14-5) the fpecific gravity of pure vapour is to that of °f vapour and * 

 air as 4- to 9. I fuppofe he compares it with air at the ufual 

 denfity of 30 or 29, and at fome particular temperature which 

 is not mentioned ; for at high temperatures the difference muft 

 be much greater, as appears by the foregoing tables. 



(To be concluded in our next.) 



