If \ : > 



r 90 ] 



of nitrous acid gas ; but if made over water, it is 

 immediately abforbed by it. 



As azotic gas does not combine with nitrous 

 gas, but remains behind when mixed with it, 

 Dr. Prieftley employed nitrous gas for afcertain- 

 ing the proportionate quantity of oxygen gas in 

 atmofpheric air. To this end equal parts of 

 nitrous gas and atmofpheric air are mixed over 

 water, and the greater or lefs diminution of vo- 

 lume of gafes determines the relative quality of 

 the air examined. 



To afcertain the purity of atmofpheric or other 

 airs, different kinds of eudiometers have been de- 

 icribed by Prieftley, Magellan, and Abba Fontana. 

 A fimple method is defcribed by Dr. Archer, which 

 is, to have a tube, between two and three feet in 

 length, one third of an inch wide, and open at 

 one end, a meafure for the airs ; a fcale careful- 

 ly graduated, according to the contents of the 

 meafure, into tenths and hundredth parts, fo that 

 one of the hundredth parts will be about one-fixth 

 of an inch. The operator is to fill the tube with 

 water, and invert it in a trough of the fame fluid, 

 and fet it over a hole on the fhelf of a trough; 

 the tube will remain perfectly full of water, from 

 the preffure of the atmofphere upon the furface 

 of the water in the trough. Then fill the air 

 meafure with water alfo, and if it be the air of the 

 place to be tried, it is only neceflary to pour out 

 the water, and the air will occupy ^he fame fpace 

 in the meafure as had before been occupied by 

 the water. Then convey the air, contained in 

 the meafure, under the furface of the water in the 

 trough, to the bottom of the tube; by gently - 



turning 



