THE EARTH. 



97 



coffin, her husband lay, who had some time 

 before died of the same disease ; and whom 

 she, poor creature, soon followed. But what 

 showed the peculiar malignity of the air, thus 

 suffering from animal putrefaction, was, that 

 the contagious steams had produced spots on 

 the very wall of their wretched apartment : 

 and Mr. Boyle's own study, which was con- 

 tiguous to a pest-house, was also spotted in 

 the same frightful manner. Happily for man- 

 kind, this disorder, for more than a century, 

 has not been known in our island ; and, for 

 this last age, has abated much of its violence, 

 even in those countries where it is most com- 

 mon. Diseases, like empires, have their re- 

 volutions ; and those which for a while were 

 the scourge of mankind, sink unheard of, to 

 give place to new ones, more dreadful, as 

 being less understood. 



For this revolution in disorders, which has 

 employed the speculation of many, Mr. Boyle 

 accounts in the following manner: " Since," 

 says he, " there want not causes in the bowels 

 of the earth to make considerable changes 

 amongst the materials that nature has plenti- 

 fully treasured up in those magazines, md as 

 those noxious steams are abundantly supplied 

 to the surface, it may not seem improbable, 

 that in this great variety some may be found 

 capable of affecting the human frame in a 

 particular manner, and thus of producing new 

 diseases. The duration of these may be 

 greater or less, according to the lastingness 

 of those subterraneous causes that produced 

 them. On which account, it need be no won- 

 der that some diseases have but a short du- 

 ration, and vanish not long after they appear; 

 whilst others may continue longer, as having 

 under ground more settled and durable cau- 

 ses to maintain them." 



From the recital of this train of mischiefs 

 produced by the air upon minerals, plants, 

 animals, and man himself, a gloomy mind may 

 be apt to dread this indulgent nurse of na- 

 ture as a cruel and an inexorable stepmother : 

 but it is far otherwise ; and, although we are 

 sometimes injured, yet almost all the com- 

 forts and blessings of life spring from its pro- 

 pitious influence. It would be needless to 

 observe, that it is absolutely necessary for 



a Keil, Robinson. 



the support of our lives ; for of this, every mo- 

 ment's experience assures us. But how it. 

 contributes to this support, is not so readily 

 comprehended. All allow it to be a friend, to 

 whose benefits we are constantly obliged : 

 and yet, to this hour, philosophers are divi- 

 ded as to the nature of the obligation. The 

 dispute is, whether the air is only useful by 

 its weight to force our juices into circulation ;" 

 or, whether, by containing a peculiar spirit, it 

 mixes with the blood in our vessels, and acts 

 like a spur to their industry. 1 * Perhaps it 

 may exert both these useful offices at the 

 same time. Its weight may give the blood 

 its progressive motion, through the larger 

 vessels of the body ; and its admixture with 

 it cause those contractions of all the vessels, 

 which serve to force it still more strongly for- 

 ward, through the minutest channels of the 

 circulation. Be this as it may, it is well 

 known, that that part of our blood which has 

 just received the influx of the air in our bo- 

 dies, is of a very different colour from that 

 which has almost performed its circuit. It 

 has been found, that the arterial blood, which 

 has been immediately mixed with the air in 

 the lungs, and, if I may so express it, is just 

 beginning its journey through the body, is of 

 a fine florid scarlet colour ; while, on the con- 

 trary, the blood of the veins, that is returning 

 from having performed its duty, is of a black- 

 ish crimson hue. Whence this difference of 

 colour should proceed, is not well under- 

 stood ; we only know the fact, that this florid 

 colour is communicated by the air; and we 

 are well convinced, that this air has been ad- 

 mitted into the blood for very useful purposes. 

 Besides this vital principal in animals, the 

 air also gives life and body to flame. A can- 

 dle quickly goes out in at exhausted receiver; 

 for having soon consumed the quantity of air, 

 it then expires for want of a fresh supply. 

 There has been a flame contrived that will 

 burn under water ; but none has yet been 

 found that will continue to burn without air. 

 Gunpowder, which is the most catching and 

 powerful fire we know, will not go oft in an 

 exhausted receiver ; nay, if a train of gunpow- 

 der be laid, so as that one part may be fired 

 in the open air, yet the other part in vacuo 



b Whytt upon Vital and Involuntary Motions. 



