SILK-WORMS. 
141 
agriculture, may greatly and speedily destroy in- 
veterate errors, and procure rapid improvement. 
Hitherto, for instance, it has been reckoned a 
good method of purifying the interior air of a 
laboratory to burn some odoriferous or vegetable 
substance, to produce a grateful smell, while, 
instead of purifying or improving the air by these 
means, they were rendering it considerably worse. 
It has been erroneously imagined that what usually 
occurs in our perception of offensive effluvia, 
should be equally applicable to the noxious qua- 
lities of the air, which, as they affect the lungs, 
have great influence on the general system of ani- 
mal life. The case is, however, dissimilar ; in pro- 
ducing a pleasant smell in a room of which the 
air is vitiated, we do but disguise to the sense the 
bad quality of the air we breathe, but the lungs 
are not the less affected. We are then mistaken 
in employing such means in the laboratory ; instead 
of correcting the noxious air, we make it worse. 
It might perhaps be useful here to exhibit some 
principles which belong more properly to physical 
science than to the art of which I treat ; but I 
imagine it may suffice to point out some positive 
facts, from which science itself has deduced its 
principles. 
1st. In whatever manner any odoriferous vege- 
table may be burnt in the centre of the room, and 
