22 LETTER concERNING 
eafes, the moifture being pure, unmixed with the poifon- 
ous vapours arifing from putrid marfhes and ftagnant 
pools, in which many infeéts die and corrupt the water. 
Thefe places only, in my opinion, (which however I fub- 
mit to yours) afford unwholefome air; and that it is not 
the mere water contained in damp air, but the volatile 
particles of corrupted animal matter mixed with that wa- 
ter, which renders fuch air pernicious to thofe who breathe 
it. And I imagine it a caufe of the fame kind that ren- 
ders the air in clofe rooms, where the perfpirable matter 
is breathed over and over again by a number of aflembled 
people, fo hurtful to health. After being in fuch a fitua- 
tion, many find themfelves affe@ed by that fedricula; 
which the Englifh alone call a cold, and, perhaps from the 
name, imagine that they caught the malady by gong out 
of the room, when it was in fa& by being in it. 
You begin to think that I wander from my fubje@, and 
go out of my depth. So I return again to my chimneys. 
We have of late many leCturers in experimental philo- 
fophy. I have wifhed that fome of them would ftudy this 
branch of that fcience, and give experiments in it asa part 
of their letures. The addition to their prefent apparatus 
need not be very expenfive. A number of little reprefen-. 
tations of rooms compofed each of five panes of fath glafs, 
framed in wood at the corners, with proportionable doors, 
and moveable glafs chimneys, with openings of different 
fizes, and different lengths of funnel, and fome of the 
rooms fo contrived as to communicate on occafion with 
others, fo as to form different combinations, and exem- 
plify different cafes; with quantities of green wax taper 
cut into pieces of an inch and ha’f, fixteen of which ftuck 
together in a {quare, and lit, would make a ftrong fire for 
a little glafs chimney, and blown out would continue to 
burn and give {moke as long as defired. With fuch an 
apparatus all the operations of {moke and rarefied air in 
rooms and chimneys might be feen through their tranfpa- 
rent - 
