VOL. LXVIII.J PHILOSOPHICAL TRANSACTIONS. 329 



ducing their pernicious effects. Moisture to a certain degree is necessary to 

 every kind of fermentation ; hence it would seem that by the falling of a certain 

 quantity of rain on marshy grounds, a fermentation immediately commences in 

 the putrid soil, a quantity of vitiated particles are set at liberty, by which the air 

 is polluted. The degree of fermentation is influenced by the degree of heat and 

 the greater or less quantity of moisture. 



Ex[}. 24. To the mud used in the last experiment, more water was added to 

 dilute it so that, on subsiding, a considerable height of water swam above it ; 

 it was confined with the air, and stood as in the last experiment. The air being 

 then tried by the test, it was in no instance found further diminished than from 

 62° to 56°. This experiment was made with a view of discovering the effects of 

 marshes and bogs when laid under water ; and we find that their danger is in a 

 great measure obviated by it : so that the putrid fermentation is either prevented 

 by too much moisture, or the effluvia are absorbed in passing through the 

 superincumbent bed of water : perhaps the cold generated by evaporation may 

 have some efi^ect. This fully proves the propriety of Sir John Pringle's remark, 

 where, in giving cautions for avoiding diseases arising from putrid air, he says, 

 " As for cantonments in marshy grounds, if the troops mast remain there in the 

 dangerous season, it will be better to float the fields entirely, than to leave them 

 half dry ; for the shallower the water the more it will corrupt, and the evapora- 

 tion will be greater in proportion." How beautifully is this illustrated by the 

 21st, 22d, 23d, and 24th experiments 1 An instance of the perfect agreement 

 of faithful observation with truth and nature. 



Exp. 25. Two ounces of dirt swept from the streets were inclosed in the 

 phial as before : after standing together 24 hours, the air was found to be 

 diminished considerably, from 62° to 50°. Hence it appears how well the ma- 

 gistrates consult the health of the inhabitants, as well as the neatness of cities 

 and large towns, by enforcing due attention to the cleaning and paving of the 

 streets in their respective districts. 



Exp. 26. The same quantity of loamy, vegetable earth, out of Dr. W.'s 

 garden, and brought to the consistence of thick mud by addition of water, was 

 next tried. The air was found but little worse ; in one instance only diminished 

 from 59° to 55°; in another, from 64° to 6l°. It is hence probable, that fine 

 loamy vegetable earth contains little putrescent matter, as it gives little noxious 

 effiuvia. The addition of animal and other kinds of manures will much vary 

 their effects in this respect. 



Exp. 27. A mass of the same consistence was formed of pure clay and 

 water, the other circumstances of the experiment being the same. The air was 

 not found the worse by it in 6 trials : in one there was only the small difference 

 of 62° to 61°, certainly the result of some slight inaccuracy. So that the pure 



VOL. XIV. U u 



