M A R S H A I R. 



4-3 



infer from the following circumftance ; marflies have no 

 noxious influence, during the winter feafon. They caufe 

 difeale when the circumftances are prefent which promote 

 putreiadtion; as, a proper degree of heat, a due quantity of 

 moifture and the contact of atmofpheric air or fubflanccs 

 capable of affording oxygen; as ■zew/tr. That a certain de- 

 gree of moifhjre is neceifary, appears evident from White's 

 experiments, related in the Philofophical Tranfadions: he 

 fays, ' ' acertain degree of moiflure feems neceifary to produce 

 the bad effefts of marfhcs; for mud when perfectly dry did 

 not alter the air." He might have added, that too much fluid- 

 ity will likevvife prevent their bad confequences, which is 

 proved by the neighbourhood being healthy when they are 

 overflowed. An overflow of water may operate by pre- 

 venting the powerful efiects of the iun. Experience 

 teaches us, that their bad eiletls are difcontinued, when they 

 become dry. Covering them with clay and other fubflan- 

 ccs not liable to putrefadion, deftroys their bad efteds, fo 

 does cultivation, froit, &c. 



Living trees being planted in their neighbourhood ren- 

 ders the fituation more healthy, by abforbing the gas ex- 

 haled during putreficlion and affording oxygen gas. 



White's experiments prove, " ift. During fixteen hours, 

 air confined in a phial over water did not fufter a change, 

 adly. Pure clay moiftcned did not alter the purity of the 

 air. 3dly. Sand moiftened did not change the purity of the 

 air." But 4th. Mud (li'Ijich coufjls of earths intimately 

 mixed with dead animal andvegctahle Jubjlances ) rendered the 

 air very impure, as I proved by the experiments -iSjhicb I per- 

 formed. 



The following refledions occurred to me fome time flncc, 

 and are copied from my note book. 



To arrive at any certain knowledge rcfpeding the man- 

 ner by which marflies can be fuppoled to affed the atmof- 

 phere, we mufl; inveftigate their compolition. 



They 



