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that air with respect to animals , and its effect on the flame of a candle . The experiments he had made having produced striking results , he proceeded to a similar examination of the inflammable air . Endeavouring afterwards to-ascertain all the circumstances in which these two airs are apparent , he soon remarked that iri various instances of combustion .
especially in the calcination of metals , the nature of the air in which the operation is performed is changed , without the production of either fixed , or inflammable air . Hence his discovery of a third -species of noxious air , which he denominated \ phlogistic ated air > and which has sin c $ keen called axoiic gas .
Me made use of small animals in trying the pernicious effect of these different sorts of air , and consequently was obliged to inflict torment on sensitive beings : his disposition is manifest in the joy he felt in the discovery of a fourth species of air , which enabled him to omit these cruel
experiments ^; this the nitrous air ^ \ vhicH has . the property of suddenly diminishing the volume of every other air with which-it is mixed , nearly in the proportion in which the other is calculated for respiration ; and is therefore a ' me 4-sure , to a certain degree , of the salubrity of different airs .
This discovery , the origin of eudiorrietry , was of the greatest importance : a measure of this kind was requisite for every branch of natural philosophy ; and it might have been made particularly useful in the practice of physic , if any scientific process were not introduced with so much difficulty even into the exercise of arts which ^ re the most scientific .
Combustion , fermentation , respiration , putrefaction , produced sometimes fixed air , sometimes inflammable air , sometimes phlogisticated air ; there existed then an infinite number of cause * tending to vitiate the air , yet its purity had not been sensibly dimini hed during the length of time in which those causes had operated ; it was therefore evident that nature possessed some method of continually
restoring that purity . Priestley found that this was effected by the property which he observed in vegetables , of purifying the atmospheric air during the day , by decomposing the fixed air 5 this property is the key to tlie whole vegetable economy , and , regarded in connection with the tendency of animals to deprave the air by respiratioh 3 ifjpartly
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shewed at that time what has since been more fully developed , that the spring of life consists principally in a perpetual transformation of elastic fluids . Thus the discoveries respecting the nature of airs , opened a new field of enquiry concerning living bodies ; they imparted new light to physiology and physic : and in a sh ' ort time that light became still xnoi e vivid .
Having applied the heat o £ a burningglass to some lime of mercury , Priestley had the satisfaction of obtaining unmixed that portion of the atmospheric air , calcu- ; lated for respiration , which is consumed by animals , restored by vegetables , and changed by combustion : he gave it the name of dephlogisticated air * The other airs which are different front
the common air extinguished lights ; this , caused them to burn with a vivid flame , and with amazing rapidity ; animals were destroyed by the others ? in this they lived longer without the necessity of having it renewed than even in the common airy and their faculties seemed to derive from
it uiiusual energy . For a little while it ' Was thought that a new mean of exciting * and possibly of prolonging life , 01 ? at least * that ah unfailing remedy for the genera- lity of pulmonary complaints , was found
out . This hope was fallacious ; but still the dephlogisticated air is one of the mo& admirable discoveries of the eighteenth , century . It is that air , now called oxygene , which is regarded by modern chymistry as the most universal agent v { nature . By it all combustion , all
calcination is effected ; it enters into the-coin * position of all the acids ; it is one of the elements of water , and the great reservoir of fire ; it affords us almost all the artificial heat employed in the arts , and in common life ; in respiration , it gives to
all animal bodies their natural heat and chief principles of motion , ; the energy o £ various sorts of animals is proportioned to the force with which it acts upon them ; there is no period of the growth of vegetables in which it does not combine with .
or escape from them in several ways ; in awoul , scareely any phenomenon in chymi try and vegetable and animal physiology can beperfectly explained without it-. I have given but a slight sketch of Priestley ' s most remarkable discoveries ; -want of time obliges me to omit a great number , which would themselves afford rich material * for another eulogy * , Eacb
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2 t $ Euogy on Dr . Priesftetf .
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), April 2, 1806, page 218, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct1723/page/50/
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