VOL. LXVIII.J PHILOSOPHICAL TRANSACTIONS. 317 



hurtful. Hence, except in particular constitutions, or in cases where their 

 stimulus may be hurtful, they may be safely used as agreeable odours, and to 

 obviate the smell in sick rooms, &c. The volatile alkali, as Sir John Pringle 

 observes, appears in this view perfectly innocent. What is here said is under- 

 stood of plants gathered and separated from the roots. Dr. Priestley discovered 

 a different property in them when in a vigorous, growing state ; they then 

 absorb from the atmosphere; but this ceases with their life, they then exhale 

 putridity, and hasten to dissolution. 



We come next to another, not less curious and important, part of our experi- 

 ments ; the effects of the effluvia from moist, marshy, and other kinds of soils, 

 on air. This subject, as particularly connected with our art, regarding the 

 endemic diseases of different countries, and a plentiful source of the most dan- 

 gerous diseases, has much employed the attention of physicians and philosophers 



Exp. 16. The air over the river Ouze was constantly purer than that of his 

 garden by 2 or 3 degrees. — Exji. IJ . The same was observable in the air over 

 the foss. This was at a time when, in consequence of floods, the current was 

 pretty rapid, all the mud and marshy ground being covered to a considerable 

 height with water. 



Dr. W. next tried what effect the same waters might have on air, when con- 

 fined together. Two ounces of the water were put into an 8 oz. phial, so that 

 there were 6 oz. of air ; being corked up, they were suffered to stand together 

 l6 hours. — Exp. 18. The air from the Ouze water was equally good as at first; 

 and this in several experiments. — Exp. ig. The same was the result in the foss 

 water. It was perfectly free from mud, yet not so clear as the river water, and 

 had some of the lens palustris swimming in it. Hence we find, that the air was 

 not any way polluted by standing over the surface of water. Perhaps if longer 

 time had been allowed in the IQth experiment, tlie lens palustris might have 

 become putrid, and hurt the air. 



Exp. 20. Some of the foss water was next tried ; so foul as to deposit a muddy 

 sediment on standing. In one experiment the air was reduced from QoP to 58° ; 

 in another, from 62° to 57° ; in a 3d, from 6o° to 56°. 



Exp. 2 1 . It has been observed by those physicians who have had the most 

 opportunities of being acquainted with the diseases peculiar to low, stagnant, 

 and fenny situations, that they seldom begin to appear till the water is so far 

 evaporated, that the black and slimy mud begins to appear. In order to know 

 this, the following experiments were made. Two ounces of the black stinking 

 mud of the foss was put into the 8 ounce phial of air ; being closed, they were 

 suffered to remain together 12 hours. The air in 12 trials being put to the test, 

 the results were as follow. In 7 experiments the air was reduced from 62° to 

 34° ; in 3, from 62° to 36° ; in others, from 60° to 35°. These are convincing 



