142 MEMOIR OF PRIESTLEY. 



fermentation, with reference to its deleterious influence on animal 

 life and its property of extinguishing the flame of candles. 



His experiments having given him some remarkable results, he was 

 induced to extend them to inflammable air. 



In aiming to determine all the circumstances in which these two 

 kinds of air manifest themselves, he soon remarked that in many 

 instances of combustion, especially during the calcination of metals, 

 the air in which such operations are conducted is altered in its nature, 

 without there being either fixed or inflammable air in the product. 

 Hence his discovery of a third species of deleterious air, which he 

 named pJilogistic air, and which has been since called azotic (or nitro- 

 gen) gas. 



He had availed himself of small animals to test the pernicious 

 action of these different products, and was thus obliged to give pain 

 to sentient beings. His character is displayed in the joy which he 

 felt when the discovery of a fourth species of gas enabled him to 

 dispense with this cruel expedient. This was nitrous air, which has 

 the property of suddenly diminishing the volume of an}'' other with 

 which it mixes, very nearly, in the proportion in which that other 

 air is respirable, and which consequently affords the means of mea- 

 suring to a certain point the degree of salubrity possessed by different 

 kinds of air. 



This discovery, which was the origin of that branch of physics 

 known as eudiometry, was of great importance ; all the natural 

 sciences were interested in possessing such a measure, and medicine, 

 above all, might have turned it to account but for the difficulty of 

 introducing scientific processes into the practice of even the most 

 scientific arts. 



Combustion, fermentation, respiration, and putrefaction are found to 

 produce sometimes fixed air, sometimes inflammable, and sometimes 

 phlogistic air. There is thus no end to the causes which may vitiate 

 the atmosphere, and yet its purity has undergone no sensible altera- 

 tion during all the time those causes have been acting. Hence it 

 follows that in nature there must exist some constant means of restoring 

 that purity. 



Priestley detected these means in discovering that vegetables possess 

 the property of decomposing fixed air during the day, and that they 

 thus puril'y the atmosphere. This property, besides being the first 

 key to the whole vegetable economy, when taken in connexion with 

 that exerted by animals of vitiating the air by respiration, disclosed 

 then, what has since been better developed, that the restorative ener- 

 gies of life consist chiefly in a perpetual transformation of elastic 

 fluids. 



Thus these discoveries respecting air opened an altogether new 

 field to the researches upon living bodies, and shed a light on physi- 

 ology and medicine till then unknown. But still more unexpected 

 and brighter rays were soon to issue from the same focus. 



Having applied the heat of a burning-glass to the calx of mercury, 

 Priestley had the good fortune to obtain, in a pure and isolated state, 

 that respirable constituent of atmospheric air which animals consume 



